Don't worry about the ride quality in the S5. It's firm but quite comfortable (perhaps the seats help) and it doesn't become unsettled on bad roads. I find it an ideal compromise and I've never had a passenger complain.
Size-wise, I'd say it's very spacious in the front, but a bit cramped for any rear-seat passengers on a long trip. But for shorter trips (or shorter passengers) the rear seats are more than adequate. The biggest issue is getting in and out of the back, and older people will struggle. Compared to your BMW you'll find the S5 feels noticeably wider when you're driving on narrow roads.
The S5 has a very linear power delivery and it doesn't necessarily feel as fast as it is, so it doesn't ever feel too much of a handful. Compared to your 330CI you'll really like the superior low-end power the S5 has. In fact you'll probably feel like the peak output of the BMW is roughly what you are experiencing across the whole rev range in the S5 - that's really the difference. On UK roads there is rarely enough space to really hold the S5 on the gas for more than a few seconds and, in any case, you're soon getting too quick for safety, so in a sense it can be too powerful to really give it the beans on the road.
Give one a test drive. The overall quality of the S5 is streets ahead of a BMW. And you'll love the noise it makes.
