My Sapphire is on the standard 15s and handles well on bumpy country roads compared to some cars I have followed and who have reported that they were really feeling the jolts on their 17s.
I did have a set of 7.5j x16s off my old XR4 which I tried for a while and the turn in felt better but I seemed to think it was worse on some bumpy roads.
I would say 16s are a good road compromise simply because you have access to a wider range of performance tyres - for example they don't do the new Yoko Paradas in a 15 ( I think).
Depends what you want to use it for really - you'll probably find the bigger wheels are good on a smooth track