both cars have huger aftermarket. all can be chipped, turbo swap, exhaust, etc.
on the 1.8T I'd suggest a 2000-2001, they have DBW and produce a bit more power when chipped, plus have been proven to be the most reliable. get quattro and a 5MT with OEM sport package, add a chip, maybe a new DV, for a very nice all-round car.
the 2.7T is a monster with just a chip, over 300 hp and 366 ft-lbs! Some revisons were made to the y-pipe and turbo oil lines for reliability on the latest models, the parts can be retrofitted to earlier models. 2.7T needs a careful eye on maintenance, stock or chipped. Exhaust makes nice gains stock or chipped on either car, esp. on the 2.7T, check out the UUC pipes, incredible sound and power!