which IS300? the first generation (altezza) or the second generation IS300? if it's the second genaration IS300 vs the new A4, i'd take the new A4. you have the diesel option plus it looks a lot better (imho) than the new IS300.

now if you're comparing the first gen is300 vs an older a4, then i'd take the previous generation is300 for modability, but of course i am bias