Hmmm....my personal experience suggests that if you choose your tyres carefully these can have a more pronounced effect on comfort than the size of the wheel.
Driving two mito 1.4s with one with michelins and one with pirellis back to back was a real eye opener: the Pirellis were a much, much more comfortable drive.
Having owned two 159's and upgraded the wheels from 17 to 18, the first from Pirelli to Bridgestone, the second from Bridgestone to Pirelli, the 18" Pirellis felt way way more comfortable than the 'made from stone' Bridgestones, although on the 17's the Pirellis felt a little squidgy. I later had a set of Michelins (on 18) and they were about half way between the two.
When I had my 147's 15" to 16" with firestones definitely made the car a little firmer. 16" Bridgestones on another (1.6) felt like stone wheels again, but a move to 17" with Kumho's felt like a move in the more comfortable direction. None of these 147's however ride like my GT.
So my advice would be go 18 if you want, but choose your tyres carefully, conversely if you want your GT to ride like it's on 18's, get some hard tyres - bridgestones maybe!
146 1.6TS, 156 2.0TS, 156 1.8TS, 147 1.6TS, 147 2.0TS, 159 1.9JTS, 147 1.6TS, 159 1.9JTDm, Mito 1.4, GT, Giulietta 2.0JTDm, Giulietta 1.4tb, Giulietta 1.6JTDm........