well the R32 is obvioulsly the daddy! but the mk4 is more tuneable!!
have a look in the classifieds on how much your mk3 is worth!

havnt heard ot the turbo's going pop!
and it all depends on what you want from your new mk4! the agu is the early one, and better engine, and with a ko3s and a few other mod's it will produce around 200bhp!