I would have to say the ride is definately firmer with the 16" wheels. This is due, mainly, to the reinforced side walls from the 16" run flat tyres that are automatically specified when you order 16" wheels. However, I the car feels so steady through corners, there is no hint of tyre roll and the steering feels very precise.
Please remember though, I have only driven my Mum's car briefly with 15" wheels and my car, with 16" wheels, for 195 miles. If you want comfort then go with the 15" wheels as these tyres are not run flats but normal tyres. They are also the cheaper option when it comes to replace the tyres. If you want slightly better handling and looks too, then go with the 16" wheels.
If I were you I would try and get a test drive with both wheels at your local dealership.
Please ask some more questions if you would like some more detail, as I am in a bit of a rush at the moment. I can give you some more info later and answer any questions you have.
p.s. Check out my Gallery for pics of my car. My Mums car is in there too with the 15" rockets.
Thanks for your reply,i think i will ask the dealer if i can try out both like you said.I want the look but dont want a bumpy ride and i have been told that the steering is a lot heavier with larger tyres.Thanks for your help happy driving!