We made a good choice
After spending time in this forum and doing lots of research, last year we bought a Zodiac 310 and a Mariner 9.9.
There will be times when a bigger boat would be better and times when a smaller one would be more convenient, but this seemed a fair compromise.
Today, on a side branch of the Ouse, near St Ives (Cambs) anchored out of sight and sound of the rest of humanity, having passed through a gap between overhanging trees and riverbank that would have stopped anything bigger than our little boat, nothing could have been better. A picnic in the sunshine, followed by jumping and swimming from the boat, sunbathing "on deck" and watching the wildlife - what a way to spend a summer's afternoon. Nothing else but a small SIB would have allowed us to do this.