what do you mean by tuning? do you just mean the usual cai, chip, exhaust? if so, then the 328 will probably be the best since its more powerful and has more displacement from the factory. then you could go further with the obd-1 conversion. but if you want to build the engine to M3 spec or have more than 3L of displacement then the 325 could be a better engine to start with since they can be had for less money yet share the same bore size.