There are a number of aspects to consider when upgrading your induction system. VW engines are notoriously under carbureted, a repercussion of the original design. A stock VW carburetor is proportionally small in relation to VW engine size when compared to most (if not all) other car makes.
Carburetor sizing theory
Your first consideration should be to note how many cylinders each carburetor throat is 'feeding'. If it's only feeding one cylinder, you have complete control of tunability since you can adjust the air/fuel mixture perfectly for each cylinder. Carb throats that feed multiple cylinders (the case in stock engines) have to be adjusted to a compromise setting, and the result is that all the fed cylinders run at a less than optimal air/fuel mixture. This is one reason why dual carbs provide better mileage than centermounted carb kits (including stock) assuming they are driven similarly.
Next you are faced with a decision on the type of carburetor set-up you should run. This is where most people could use some clear-cut details and general clarification. Choices include:
1. Centermount 1BBL
2. Centermount 2BBL
3. Dual 1BBL (Kadron, Solex Weber ICT, Dellorto FRD)
4. Dual 2BBL (Weber IDF, IDA & DCNF, Dellorto DRLA & Solex 40P11)
Jetting for centermount vs. dual systems
A key difference to note between a centermount application and a dual carb set-up using the 'same' carburetor is that, by comparison, the carb in the centermount is jetted slightly richer on the idle circuit. This is because of fuel condensation issues. The gasoline has a hard time staying as a vapor on the long run to the cylinders in a centermount set-up. You must take into account that carburetor jetting requirements differ slightly for dual and single applications when dialing in your set-up.
Centermount 1bbl systems
Stock: As you might expect, a stock carburetor will make your vehicle perform as it did when new, assuming that the rest of your engine is also up to the task or in almost new condition. Many vehicles have carburetors that are simply worn out, that have been ignored to a point far beyond what could remotely be considered 'acceptable condition' by reasonable standards. Wear commonly results in a vacuum leak at the throttle shaft bushing (air sucks in around the worn bushing). Quality machine shops like RIMCO can repair this condition, at a reasonable price, but some carburetors (specifically the 34 PICT) just don't rebuild well, and you are better off to replace them. Stock carburetors are still available new from many vendors, and the installation of a new carburetor on a well-maintained engine can transform your bucking vehicle into a smooth operator.
Stock Replacement: As you probably guessed from the name, replacement carburetors are not original equipment. But, they are still appropriate for stock-type use. The Solex 30/31 is one of these carbs. The Solex 30/31 is jetted very lean, and often require a main jet upgrade to get them to run properly. Quality control for replacements is not as good as that which was on the original Solex carbs, so you take your chances and should expect a lemon to pop up now and again. Make sure you deal with a vendor who will stand behind their products in case you get one of the lemons...
Weber also made a replacement carburetor, and it was quite popular and available in the 80s' - much more so than now. There are still a number of these ones still floating around, but if you plan on disassembling this particular carb, you'd better hope that you have as many arms as an octopus, an IQ over 130, and the dexterity of a brain surgeon... and I'm not kidding!
Centermount 2bbl systems
Progressive carburetors used on VW's are mechanical secondary carburetors. A mechanical secondary carburetor opens in relation to throttle position. The Progressive set-up uses a small primary barrel, allowing for excellent drivability and mileage. These also offer a larger secondary barrel for more power when you open the throttle. These carburetor kits can take a lot of time - up to 8 hours - to dial in for your particular car. This is partly because they are supplied from the factory with generic jetting often not suited for the VW engine, and also partly because the jets can be difficult to access. A good kit won't be too far off straight out of the box, but even a close one will need some tweaking before it is perfect! The centermount progressive is a fantastic carburetor once it's jetted properly (but remember that it's no small task). Most are also available with an electric choke, making it easier to start and drive when the engine is cold and when air temperatures are colder. Most stock carburetors came with electric chokes, and they definitely made life easier for the driver of a vehicle with a centermount set-up. The Progressive set-up is an excellent combination of performance, drivability, and economy. The main downfall of this system is the time it takes to get it set up properly.
Non-progressive systems usually use a Dellorto DRLA or Weber IDF or DCNF 2BBL carburetor. The Non-Progressive carburetor set-up tends to sacrifice drivability and economy in favor of performance. You may hear about the Holley BugSpray carburetor periodically. It was a decent performer in it's day, but is essentially obsolete due to age and wear, and you aren't likely to find one in good enough shape to rebuild or run.
Centermounts and manifold heat
All centermount applications require intake manifold heat to perform properly in cool weather. Original VW carburetion relied on it because centermount set-ups can ice up if manifold heat isn't available! Manifold heat also 'assists' mileage and makes centermounts considerably more driveable, since fuel is more likely to stay atomized, since it is not as likely to condense on the way to the cylinders. Be careful when buying your carb kit, since many kits have inadequate manifold heating, or none at all! Proper manifold heat will make or break your set-up when it comes to functionality and drivability. A 'bargain' kit is hardly a bargain when it interferes with your driving experience by making your car temperamental! I strongly suggest you pay the little bit extra for a good kit, unless you live in the hot desert where the manifold heat won't be needed as often.
Type 3 and 4 vehicles
Because of the need for manifold heat, I do not recommend centermounts for the type 3 or 4 engine, since the kits available for these engines do not have provisions for manifold heat. For the type 3 or 4 engine, I recommend dual carbs (more info to follow). Also, the exhaust system you have on your engine will affect manifold heat. Many headers do not have the proper intake pre-heater flanges required to accomplish adequate intake manifold pre-heating. Even headers that do have the pre-heater flanges must be manually drilled through to make them functional. Don't assume yours is ready to roll out of the box!
Off road applications
90% of the time, off road applications should use a centermount system. Why? There are a couple reasons. The first is simplicity. Dual carbs require more complex linkage, and when you are talking about off-road use, this is just more parts to break or give you problems. In addition to this, dual carbs are mounted on the outside of the car, where they are vulnerable to getting hit with debris. Lastly, a carburetor mounted in the center is going to get bounced around less then if it's mounted on the outside of the car as the car is pitched side to side. Therefore, fuel control is easier (less chance of flooding) with a centermounted carb. The power difference between a centermount and dual set-up is very small, and with the above noted advantages of running a single, it's an easy decision to justify. If you want corroboration, just have a look at the winners of the SCORE races and then check out the induction systems installed on the winning cars.