Congrats on the job offer!
I would take the earlier class date unless you have something pressing you need to take care of before late May. I'm not familiar with how new hire seniority works in the new RAH structure, but I assume you would still enjoy all the benefits of being a few numbers senior if you took the earlier class date (bidding, upgrade time, etc.).
That being said, if you feel strongly about flying the "big" jet, wait it out. A few numbers most likely won't make much of a difference, but you never know
On an unrelated note, can 145 FO's bid into the 170? What about for upgrade? Can a 170 FO bid back to the 145 (or vice versa)? Any insight from someone "in the know" would be appreciated!
Finally, did you get any time in that Widgeon you've got on your avatar??? Looks like a sweet bird!