Slow down...let's not talk turbo kits until you have a car. Suffice it to say it's gonna cost around 3k + install for either car for a decent turbo kit and leave it at that.
How about we choose a car and buy it, and see how you like that power, and go from there. I will start by making these points:
1.8T:
-Decent power
-Cheap to upgrade to a moderate level of speed
-Huge aftermarket for this motor
-Pretty good gas mileage
-Fairly reliable except for vac leaks and coil problems
-Require a little more care than a VR6
-The 337 will require little to no exterior modification since it's already a pretty good looking car
VR6:
-Very torquey motor about about the same power as the 1.8T
-Bolt-ons are not too expensive but not as beneficial as 1.8T mods
-Large aftermarket for this motor as well but mostly expensive turbo/supercharger kits
-Pretty good gas mileage for a 6 cylinder
-Very reliable except for a coolant pipe
-Require less attention to detail and less sensitive to oil changes and fuel used
-Has a timing chain unlike the 1.8T so it lasts much longer before requiring change/maintenance
-Will need some sprucing up of the exterior to make it look as tasteful as the 337
Basically the VR6 is gonna cost you a little more if you plan on doing much for upgrades (for power or appearance) but any performance upgrades (as far as a turbo goes) will yield greater results.
The 1.8T will make for an easier "tuner" car and will allow you to focus solely on engine performance since the exterior requires little to no modification to look clean.
Try to think practically when buying a car. Don't think "well if I spend 10,000 dollars on this one car will be better than the other," because from experience I know for a fact that rarely do people actually follow through on their plans for their cars or at least not immediately after buying it. Shit comes up and you gotta shell out cash for it..that's life
Hope that helps
-Cody