Actually if performance is your goal stay away from the S4 manuals. The gearing in the S4s isn't great. The 82-85 Euros and the GT/GTS had the better final drive which made them more fun at lower speeds. All the 87-89 S4s I have driven in manuals have been dissapointing because the 2.21 R&P just kill first and second. Now if you just want the newest 928 you can afford and like the newer looks go for the S4s. They have that updated timeless look. However, if you plan on supercharging it like some of the guys are doing now you would probably want the S4 for the taller gears.
As for the cost of parts, I would say with the big three (928 intl, 928 sp, and Devek) the cost of owning a 928 is coming down. Not to mention there are a few savy 928 owners that are either making parts themselves (and reselling them) or have found many aftermarket adaptable replacement parts. As long as you or your mechanic sources parts through one of those venders the costs are about on par with 911 parts. If you go to the dealer you know the story. Just like any other commodity once the supply exceeds demand the prices fall.
BTW one of the guys on the 928 list just dynoed his stock 87 S4 with a vortec charger and turned 512hp at the rear wheels. I had a ride in a similar car last year and lets just say I was impressed.
Edit for P Porn
