What year are they looking at? If its the older body style (93-99) get the VR6. If its new probably wiser to get the 1.8T if your looking for cheap power. Turbo upgrades for VR6s run about $5k or so (sometimes less). Superchargers run just over $3k.

And there is only about 100lb difference between a Jetta and a GTI so its just a matter of which body style you prefer.