Ahhh, the 717 seats . . . . they are comfortable . . . . compared to sitting on an upside-down milk crate, perhaps. It is a great airplane overall, though, and I miss those handy-dandy fold-down desk thingies.
Which aircraft to bid as a newhire?
In the 717: you'll move up each time someone upgrades in either airplane, or when a newhire gets 717, but you'll be one of 500.
In the 737: you'll only move up each time someone is hired below you, at least until the first batch of 737 FO's become eligible for upgrade (in another year) but you'll be one of approx 120.
Since there are many more 717 FO's (and almost all future deliveries are on the 737) I would say your relative seniority as an FO will improve faster on the smaller 737 fleet.
The 737 captains will be becoming more junior as time goes on (and senior 717 CA transition), so you will find the opposite to be true as a Capt.
In other words, I would say that the 737 will be better for FO's, but you would have a better QOL upgrading on the 717 . . . . so if you are about QOL, bid 737FO and upgrade 717. If you would rather build experience in type before upgrade, the 717 is probably your machine.
Either way, welcome aboard. It has really been a pleasure flying with all of the quality pilots we have been hiring . . . . With the upgrade running at 2.5 years right now and probably decreasing to 2 years soon, you should be back in the left seat PDQ.