As someone above pointed out, the E 175 is not usually considered a regional jet. It is still 2-2 seating like the regional jets, but it has a wider cabin. In fact, as I am a large sized person, I specifically seek out 175 and 195s, as their economy seats are actually wider than larger planes such as the 737 and A320. However, in First class the first class seat in a 175 is an inch narrower. So for seat comfort, First in a 175 is worse than first in a 737, especially once you consider in the curvature of the wall.

In your case I would choose the 175, A side. But if you do decide to go coach, I would still choose the 175 over the 737, and choose an aisle seat. You will only have one person next to you, plus a larger seat.