In its current form and most of its past forms, the Aviate program is not actually a flow like the AA wholly owned regionals have, but rather a preferential interview program. Currently how it works is, if you pass the Aviate interview, you are admitted to the program, and will be eligible to flow to United after you accumulate 1,600 hours of PIC time at the United Express partner. However, just because you're eligible doesn't mean you'll be offered a class date any time soon. You can also be removed from the aviate program for training failures or too many sick calls.
One reason why the Aviate program is not worth making a lateral move for, as the OP was considering, is there's no guarantee you will be admitted to the program. I'm not sure what the interview success rate is now but it used to be pretty low when I was at C5. And has been mentioned many people have been removed from the program after being accepted, though i believe most of those were in the brief period when it was a true flow.