She might like the G20 just fine on a stress-free square range when she can shoot deliberately. What will her performance be like under stress, time, and in a more dynamic environment?
^This^
I know lots of folks who think they can shoot caliber X, but this is on a square range during good weather when the birds are singing and fluffy clouds float through a blue sky.
If you can’t shoot it one handed, in the rain, using a flashlight in the other hand because it’s dark, from the ground since you managed to slide in the mud onto your ass during your gunfight, well, then, you should find a gun that fits you better.
That being said, I have taught chicks to shoot who could actually handle a larger griped pistol than I can. My hands are XL when buying gloves, and I’m an 6’2" XXL sized guy. What I noticed is that although my hands might be bigger, my grip isn’t able to wrap around as much of the pistol grip due to my hands being beefier than many gals’ hands. I know a gal that has hands the same size as mine when we place them over each other, but she can get her trigger finger onto the trigger of a standard G17/22 all the way to the second joint. She shoots G21s just fine and they fits her hands well, but unfortunately she can’t carry a .45 at work due to department policy. She now runs a gen 4 G22 with the large grip installed as it fits her hand better.
Just throwing all that out there just in case this is a similar case.
If your gal likes the 10mm, and the large guns actually fit her hand, I’d look at a gen 4 G19 with the large grip installed, or a G29/30. If you go 10mm you can always get a .40 barrel for cheaper practice.
G20 (whether SF or not):
PROS - Great muzzle velocity, excellent terminal ballistics, comfort (I also prefer the biggest framed Glocks).
CONS - Cost of ammo, recoil can get to you after a couple of hundred rounds, availability of ammo, harder to conceal, heavier to lug around, hard to find magazines.
G19 or 23 (regardless of generation - as compared to the 10mm)
PROS - Manageable recoil especially in the long term, affordable ammo, availability of ammo, more manageable size and weight, magazines are more plentiful and available.
CONS - Neither is a 10mm but with the current design of self-defense ammo, this has been marginalized.
You are wise to take your time in deciding. Impressions can change over time, and renting the same gun a few times can tell one whether it will work or not.
The G20 is a big gun to hold onto. I would not write off 9mm. Some of the +P and +P+ loads are quite hot, and have a good kick to them. The G19 is a very nice size. I have two G17’s, and will be adding a G19 soon as it fits my other half better than my G17 does, and we both like shooting the G19 a lot.
Another thing to think about when looking at the G22 and G23. If you are shooting the 180 grain loads, take a box of some .40 in 135 or 155 grain bullets. They have a lot more kick to them.
I have shot a G17 and a G22 side by side, and a week later shot a G19 and a G23 side by side. We were using PMC Bronze 180 grain in the G22 and G23, and I could not tell the difference been the 17 and 22 or the 19 and 23. Pick up some hot loads in .40 and see how she likes the .40 then.
Keep in mind, 9mm in +P and +P+ can be just as hot as the milder .40 loads, which is why I think 9mm should still be in the mix.
To me, the G20 would make a fantastic range gun, but the ammo is hard to find, and quite expensive.