Will have to go against the grain here.
Owned Gen3 21's and all were sold and replaced with 21SF's.
Bought a LE Gen4 21 in hopes of loving it, 1400 rounds later I didn't and sold it.
Sans any backstraps, they are exactly the same size. It's my opinion that Glock rushed into the Gen4. To me the backstraps look like a poorly designed after thought.
And I actually prefer the recoil impulse of the Gen3 to the Gen4, I'm talking specifically the 21's.
As far as the mag release, been shooting competitively for 25 years, with Glocks since 1994, the regular old mag release has served me well.
These of course are just my personal opinions.
Retired Test Pilot