Not sure I understand the question...

A junior base is good in that your schedule and QOL will be better (unless you haver to commute). Being an FO in a junior has nothing whatsoever to do with your upgrade opportunity.

Well, if you actually move to your base and want to avoid moving again when you upgrade, a junior base would be good because you could upgrade sooner in the town you already live in. But do you want to live in a junior base? They are usually junior for a reason...

Also some bases are real junior for FO's but very senior for CA's...this is common in expensive California towns because the FO's can't afford to live there.