I've been to the Montreal tournament once (2003) and I've been to Cincinnati the last five years (but I'm not going this year).
The field should be pretty much the same. But I would guess it's slightly more likely that Federer will go to Cincinnati than Montreal. I don't know for sure, though.
Montreal is a much nicer city than Cincinnati (or actually Mason, Ohio). There's really nothing to do in Mason outside of the tennis. But if you're driving, it's probably more convenient because you can drive from your hotel and park at the venue fairly easily (I had to take the metro in Montreal because I didn't have car). It's also really miserably hot and humid in Ohio in the middle of August. It probably isn't quite as bad in Montreal.
I haven't been to Montreal in six years, so maybe things have changed, but I recall it being a bit more relaxed there. They let me bring some fruit into the complex and the ushers didn't make me show my ticket every time I went into the stadium. Cincy seems to be a lot stricter about bringing food in and sitting in your assigned section.
Hope that helps.