FWIW I'd go for some 28's. The speed difference between 23's and 28's is quite minimal, IMO.
When you get past 28's, IMO the difference is not so much the width as it generally is the type of tire you're dealing with. I.e. road tires are 21's to 28's, mountain tires are wider and tend to use different types of tread, different compounds and so forth.
I'm using the Continental Grand Prix 4 Season in 700 x 28c's. Haven't done a full-on tour with it, but speed and handling are decent & so far it's done well resisting punctures.