For me, I chose fit and fun over displacement. I recently chose a 400cc because feels good to me and is basically what I need to get to work and have fun on the weekends.No, it's not the same as a 600, but I am learning that that is ok too, because I am having loads of fun on this one.
For me, size (my size) and budget were big factors. Of course, all those other facrtors like experience, passagers, and types of riding will play a role in the final decison. But I think that would answer for each have to lie on the person asking the question. For instance, how often do you plan on taking passengers? Someone might not mind touring on a 250, depending on other factors again, like their own comfort thresholds.
Red, you asked specifically about speed, "will it be fast enough?" If your interest is in the speed of the bike, I'd focus on that aspect.
(ps. I haven't even had my 400r for even a whole week yet, so I reserve my right to change my opinion. *smirk*)