Everyone is going to have a different answer about what's best. You should probably listen to a few different people's opinions and decide what's best for yourself. Personally if I knew I was going to be a commuter I would probably take the Brasilia so I could hold a line faster. That's what I did as a new hire and I don't regret it. You'll hold a line in LAX, FAT or SFO within a few months and those aren't bad commutes from PHX. If you start on the RJ you'll end up with a really long commute from MSP or ORD, and you'd be on reserve for years in LAX, SFO or FAT. Commuting to reserve is terrible.

The other train of thought is that if there's a lot of movement you might be able to get PHX a lot faster if you start on the CRJ as opposed to dealing with a 1 year seatlock on the Brasilia. Also they award transfers before transitions so if you were MSP based on the CRJ you would get a PHX transfer award potentially before a senior Brasilia pilot. The problem with doing it that way is that if the music suddenly stops you'll be stuck commuting to reserve for years. If you take the brasilia and everything shuts down at least you'll be commuting to a good line.