You have to factor in how many seats are available after F seats are sold. That only leaves X number of seats for upgrades. I don't know if there is a disparity with people booking F on mainline vs. 175's but there maybe. For our discussion consider they are equal.
Say 4 seats sold in F on each. You got 8 seats for upgrades on the 175 vs. 12-16 on the 737's after there. Outside of GS, I love my chances as a 1k usually on a mid fare economy fare. Usually 3 hours or more I use a a RPU. If it shorter then if the CPU goes through great.
So looking at is like the whole F cabin is open is not reality as we know, thus the more seats the better taking the above into account, IMHO.