Since it sounds like you haven't yet started coding, I would argue that the most important question to ask yourself is which framework makes you most excited about jumping in and coding something now.

Since both frameworks are Ruby, I'd hope that it wouldn't be too big of a deal to decide to change from one to the other in the middle of the project, unlike a full blown language change.

Me personally, I came from the Python world and started out with Ruby on Rails. But I didn't spend enough time with it to ever get out of the stage where it felt like I was working with a black box.

I tried Sinatra on a project and quickly "clicked" with it and I haven't used RoR since.

My biggest project runs on Sinatra, but the caveat there is that probably 90-95% of the current code has nothing to do with the framework.

I have seen the number of files grow to the point where I start wondering if I would've been better off with Rails where there would be a specific location for everything. On the other hand, I get to decide where stuff should go, and more important to me is that everything that is there is something I put there. Whereas, with Rails, I found I was wasting time trying to wade through all of the skeleton files to find where I had actually written some code. But again, I chalk that up to the fact that I never moved out of the black box phase.

As to reinventing the wheel, depending on your motivation, reinventing the wheel isn't necessarily so bad as you'd learn a lot along the way. And if you enjoy it, then I'd argue that would be more important than being half hearted on something where gems already exist for what you want.

So again, I'd say to pick whichever one motivates you to get started now.

Good luck.