Having owned both a Gti Ed30 and an R32 i think i can probably help here ...
Ed30 - easier to tune for bhp, lighter. Cheaper tax, insurance and slightly better on fuel (i averaged 29mpg). However brakes are very poor, it sounds like a dishwasher and it understeers for england when pushing on. Ed30 is also pretty expensive for the age due to the 'scene tax' and most are quite poorly specced (esp. the Gti).
R32 - not really tuneable, heavy. Expensive tax, insurance and mpg is generally worse (i average 27mpg). However, the brakes are considerably better than the Ed30, its sounds incredible and imho handles a bit better as you can use the rear end to quell the understeer slighly (more so with a Haldex controller). As most people are afraid of the perceived running costs, they are much better VFM than the Gti and usually come better specced.
My opinion?
If you want to extensively modify (esp. for power), buy a Gti. Plenty of bits to change, that imho, really do need changing.
If you want a nice sounding daily/cruiser that you dont have to pour money at, buy an R32. Its a hot hatch alternative to a 330i.
Compared to something by RenaultSport, neither are paricularly good drivers cars, so id prefer the one with the engine that suits the lazy character of the car, which imho is the R32. This is a Gti forum however, so most will disagree