A 5-door Mustang? I hope you're joking, at least about the Mustang part.
Well, based on the fact that I live in the US, I've never seen (in person) or been in an S-Max, so I don't know what that vehicle is like. Also, the Flex was on a rotating stand when I saw it, so I haven't been in it either, so I'm probably not the best person to try to explain differences, etc.