Easy answer... go for the bib

Even though they look ridiculous when your not wearing you jersey they have the advantage of allowing the chamois to stay exactly where you want it. Also, it saves you having to keep pulling your bike shorts around to readjust which can get really annoying.

The only reason they will be hotter is purely because they have more 'material' in their make up, however, the reality is that the difference this will make regarding heat retention would probably be minimal.