If one had to choose between the two, would it be better to go with a grip that's sightly under or oversized?

Under.

Specifics?

The GEN 5 Glock. Trying to figure out if I like the slightly undersized, relative to earlier Glocks, grip. Adding the smaller back strap makes the grip 1/16" larger than my Gen 3 Glock. I have good heel to heel contact with my hands on both grips. Obviously a little tighter on the smaller.

Gen4&5 “M” should feel identical to Gen3.  Plus or minus most can’t tell.

Definitely use the smaller first.  An easy to maintain grip with the smaller grip is easier to use through a lot of shots in a shooting day.

I have a g17.4 and I have tried with no back strap, med beaver tail, and large beaver tail.  I have settled on the large beaver tail as best for me, followed by med beaver, and then no back strap.  For ref i wear a xl mechanix and the glove fingers are in the verge of being too short.



With larger grip, It feels like I have a easier time gripping harder.  Also feels like I get better recoil control with the larger grip, most likely because it easier to grip harder with large grip.    With no back strap I get some fatigue in my knuckles after holding for an extended period.

Larger, more surface area for my support hand.

I'm going to have to do some shooting with and without the back strap to see what works best.