What sort of tour are you thinking? I've done three or four bus tours with Trafalgar and Globus, all were excellent. The ones I did were about ten days each, but they have longer ones, too. I've done Italy, France, Britain, and Portugal... Italy is my favourite, but all of Europe is really gorgeous.
If you're going for a short trip, I would suggest sticking to one country. The "European samplers" sound great on paper, but you need time to really appreciate each different place you go. Ten days in a country is a nice amount of time.