I don't think any Reserve unit would look at being a regional pilot in a negative light. The regional isn't really going to be "fighting for your time," you'll just drop mil leave when needed to meet your Reserve commitments (including training and initial seasoning, which I don't believe count against the five year total of mil leave you're allowed to drop without any repercussions).
I do think that you're probably better off just getting onboard with a Reserve unit ASAP. Work for a regional in the meantime, but I wouldn't delay a shot at UPT just to fly an RJ. Better off to go to UPT, then do regionals, and get to the majors as soon as you can.
