im biased towards the 1.8T, just to start off.
vr6: heavier over the front axle, na motor (as in smoother accelerations), high initial price to get big hp numbers
1.8T: lighter overall, turbocharged=quicker accelerations IMO (but then mines not stock...). low initial starting price.
what i mean by initial price is that 18ts need about $1000 to reach 210 hp (chip + exhaust). however, any other mods to increase hp after the initial investment costs alot (ie. big turbo $3000, an so on). the vr6 on the other hand, has a high initial price (supercharger = $3000), then the remainder tapers off (chip= $300, pulleys, etc)
either way you choose, you wont go wrong, but if you want mileage get the 18t.
as for auto-x's, the vws ive seen at auto-x's tend to be more 18ts, since you dont need huge hp numbers to go auto-xing (just drop a chip and exhaust, use the rest of the bling on suspension)
mk3s are actually better on the track than mk4s IMO.
if i were you, i'd get the 18t (o wait i have one already
). it fits all your criteria, except maybe reliability. but then if you wanted that you'd buy a japanese car