The 928S4 is not bad for maintenance costs unless you are paying a shop and do not have access to a shop that knows the 928. With the new timing belt tensioner option they are pretty darned easy to live with in that regard also. Yeah OK they have twice as many plugs, wires and distributor caps but no balance shafts, balance shaft belt, balance shaft seals, variocam ramps, <acronym title='dual mass flywheel'>DMF</acronym>, etc. I am not sure which car is "better," that probably has a lot to do with the class you want to run in and how much modding you intend to do. It'll be fun either way.
-Joel.