I had a 250 here in Colombia for most of last year, and part of this year. In the cities it was perfect (for me). But on those rare occasions when I ventured out, I found myself wishing I had a little more engine underneath me. 98% of the time I was in the city, so it really wasn't that big of a deal for me personally. It all depends on what kind of riding you like to do. You will definitely have more time to soak in the journey as you go, since you won't be breaking any land speed records. At the same time, I was amazed that I was usually able to stay ahead of, pass, or what have you, most of the traffic I encountered, even on a 250. Except of course bigger bikes, and the occasional Mario Andretti bus driver. But since you're going to TDF, I would vote 650. Your riding preferences may be different than mine, but if I was making that trip, no way in hell I would be doing it on a 250. It has been done before by other people, but no way in hell would I personally do it on a 250.