There are pros & cons, as well as variables regarding engine requirements & combonation.
Firstly a set of 36 IDF's will be first prize, but if they are not new and every man and his dog has had a go at them (stuffed them up) because they probably have issues, then you waisting your money, also just bolting these onto a stock motor with no further immprovements then you waisting money same applies for any other carb upgrade.
36 IDF's will be cheaper than a set of new 40/44 Solex Kadrons, or any other NEW carb setup.1 (unless you can get them used as well, Merlin has a complete good set up for sale)
If you have a stock fan housing with heater outlets the IDF's will not bolt on without some work on the offending fan housing, Kadrons will bolt on, and have much more space around them to do some work, IDF's are a tight fit in a beetle engine compartment.
Kadrons are very simple carbs easy to setup, IDF's need a litle more TLC. You cant beat the growl of a set of IDf's, DRLA's, awsome.
If it were my engine with a few improvements IE: few exta cc, say 1641cc a mild cam an extractor exhaust, stock heads cleaned up, I would go with the Kadrons, as the IDF's will only be better above about 4500 RPM, Kadrons will be more responsive off idle.
But if you plan bigger upgrades ie: more cc, headwork etc. then the IDF's
I have used both and they are both good setups.
One last thing, don't overcarb, and most important if either of these setups are not setup properly they will run worse than a stock carb, and the filling station think you driving a V8
Hope this helps you decide.