All things being equal, I'd choose the G21 gen 4 over the G17/34 gen 4. I already have all three, so it's not a matter of choosing which one to buy, but rather which one to shoot and why.
I shoot the G21 and G17 just about the same, so no help there. One might think, "Why worry about it, shoot both." Well that was my first approach, but yesterday I shot the G21 followed by the G17, with the stock dot connector, if you can believe that.
I found something disconcerting. The difference in grips sizes affect my indexing of the gun. If I'm just doing slow fire stuff, it matters little, I can adjust. But when I'm trying to hit a 3" dot at five yards at a rate of fire of 2 shots per second, and keep the hits in the dot, everything matters, including the grip and how it positions and returns the gun to the correct position on the target.
So the dilemma is, which one do I go with as my main gun? I've done a lot of pondering over this. I'll never be as good shooting both as I can be shooting just one most of the time, so which one?
I'll be honest, I've been a 9mm guy most of my life, but I now trust the .45 ACP more. Save your typing time and energy - I've heard all the arguments at least a dozen times.
OTOH, I shoot a bunch, check my signature, and .45 ACP is at least 50% more expensive than 9mm. So that has to be a consideration. And right now I'm working hard on accuracy at speed, and that favors the 9mm a bit, although not as much as I expected, well between the G21 and G17. Still there is some difference.
Anyway, and this tugs at my heart strings, I've decided to go with the 9mm/G17/G34. Plus I had to pick up and carry 1,000 rounds of 185 gn. 45 ACP in an ammo can out to the car and I wasn't sure if I could do it in one trip. Well, to be candid, I did have 1,000 rounds of 115 gn 9mm in the other hand. One arm is now longer than the other and I walk to one side now.
Anyway, that's my story as it stands right now. I just didn't realize the grip size, which fits my hand just fine, would play such a role in shooting.