reliabilty-wise the S4 is as good if not better than a WRX.
given the choice of having the MAF sensor going out or having the transmission go out at 25000 miles... well... you know...
in addition to the MAF problem there was one other problem that i've heard crop up about turbos crapping out ... i forget the specifics though and it was rather rare. check the S4 forums on http://www.vwvortex.com/
and ask them... they are pretty helpful.