Here are some of the things I would look at:
JB origionally projected E190 Capt upgrade for new hires at 6-10 months. That was based on 100-120 current A320 FOs ugrading to E190 Capt. That number is now over 140 A320 FOs upgrading to E190 Capt. Since the brief plan was to upgrade 12 Capts per month, the upgrade time for new hire E190 FOs will move to the right and be 14-18 months. Not to mention any operational, aircraft delivery, training delays that may creap into the system. That is a lot longer time that JB origionally predicted.
For a pure $$$$ look at it, the A320 may be the better choice for the short term and mid term look. The big factor would be current A320 Capt upgrade times. I think (not sure) that current new A320 FOs can expect approx 3 years to upgrade to Capt. Then put on top of that, 1 year of reserve and you get 4 years till you hold a line in the 320. Maybe the way to go is to do the A320 FO for 2 years, then do E190 Capt for 2 years (both type locked from training), then come back to the A320 as a Capt and go straight to a line. This would reduce your total time on reserve and improve your quality of life. Again, this would all depend on upgrade times in each aircraft, but something to think about.
I would expect the E190 schedules to be real crap for the first year to two years. JB's first 2 years in the A320 were also pretty bad due to the limited selection in trips. Where the 320's current schedule is a little more mature and thus a little more predictable.
If you are in a JB city and fly the 320, you could bid to layover at home, where the E190 will be very limited in where it goes. So that my not be an option at most JB cities..
If you are lazy and don't lie 6 legs a day (JFK-BOS), then don't do the E190. You can bet that for the first year, most parings will have lots of this segment in the lines.
Right now, from what I understand the difference in class dates (thus seniority) betweens types is not much. So going for seniority vs initial pay on the 320 may not be a big issue. This may change later, but is not a big thing right now.
Overall, if the class dates are the same (or close), I would go for the $$$ first, and that means the A320. I see more advantages than the E190. If the class dates are 3-6 months apart, then maybe the 190 would be better. You'll have to get out the spreadsheets and pay scales and do the math. Good luck.
Just my opinion....
FNG