first off... weight difference between vr6 and 18t is minimal. fuel economy on a 18t will never match the tdi, likewise for vr6. if you want brute power, get the vr. if you want a nice torquey experience, 18t. if you want fuel economy then get the tdi.
mod the tdi and make as much torque as a chipped 18t
i have a 18t, so personally i'd say that 18t > everything else.
average mileage i get is around 25 mpg (city & highway combined) if i use 91 octane (flame suit on) i can get up to 32 mpg (but thats also cuz my motor's not exactly stock...)