Auditioning for a show knowing you will be quitting one of them will likely get your blacklisted. An actor would have a hard time getting into another one of my show after doing that. How will they know that THIS time, they can trust you? If I can't trust you, I can't cast you.
There are pros and cons for each show....but truly, no one can make the choice but you. I will say, 6th graders in rent sounds horrific....and you will have GADS of opportunities to do that show again, at an age appropriate time for the whole cast. (And, at least in my parts, RENT is done a helluva lot more than Fiddler.)
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.