For a stock 2.0-litre engine, stock Solex PDSITs are the only option as there no practical replacements. As Barry says, never use a single centre carb as it won't work. Those hottie aftermarket IDFs etc would work if the engine had the cam, porting, big valves, free exhaust and so on to match. But on a stockie - nope. Expensive overkill, and no performance advantage for the RPMs a stock engine pulls. Plus they don't have chokes - how will they start up now that it's cold? And being twin carbs, you'll be forever trying to tune them.
The stock system works best because it is designed for the engine. It's not really a 'twin carb' system at all - it's actually a single twin-throat carb that is divided into two! The left and right carbs are different - the left is the 'master' carb and the right is the 'slave', and they 'talk' to one another via a carefully designed system of balance tubes - even the air cleaner is part of it. The stock system has not one, but THREE idle circuits that all work together. The beauty is - if it's in good condition and set up properly - it keeps working and you can forget it.
The problem is getting it set up. Normally the only thing that goes wrong is that the throttle spindles wear - there is slop at the carb when you open the throttle; sometimes you can even wiggle the spindle up and down. The solution is to get them rebushed - any carb specialist can do that for you, and replace the diaphragms/gaskets at the same time.
A detailed story of how to properly set up the stock system is here - which I hope answers all your queries.
http://www.ratwell.com/technical/DualCarbs.html
Persist with the stock system - VW knew what they were doing.