It's really a question of personal preference. I have both. I like a structured Kelly in 28 but not 32 (for me personally). A softer Kelly I could do in 28, 32 and 35. It also - again for me personally - depends very much on the leather/colour combination that the bag comes in.
In using the bag though, there is neither pros or cons between rigide and souple Kellys. The rigide Kellys tend to keep their shape better over the years, the souple Kellys tend to 'relax' a bit, but that is often a desired effect.