I felt the different sizes at a SIG Sauer store in Chandler, AZ. I didn't buy same day, but I was there and they had nothing but SIG stock there wit all the frames and colors for the 320. I liked the medium, which is the easiest to find pre-installed. Medium is usually my size. Medium gloves (except for medical nitrile gloves which I'm a Large), medium pistols, my hands are pretty "standard" I guess.
I think I made the right choice FOR ME with the P320 Compact. I still like it more than these newer guns, but based on my above posts I'm sure you can all see that I have a tendency to dislike minor things. I love that while the 320 has modularity, it's also very simple. I never liked that guns with replaceable back straps relied on a pin for their structural integrity, something I viewed as a possible failure point. 320 addressed that. I never really liked trigger safeties. 320 addressed that. And it appears to be the easiest to frame swap, and I kinda want an OD and FDE frame for days when I feel like changing things up. This is just my preferences though.
Just for the heck of it, I'll go over my impressions of these newer guns one by one since I just looked at them all side by side last night.
Beretta APX: Best grip shape of the 3 in my hand, very slim. Good trigger. Second best trigger of the 3. Good grip texture, not too aggressive or too slick. Nice low bore axis for those who look for that (I don't take much stock in this because I'm not a pro shooter and have never found it to change my shooting or recoil control). It's weaknesses in my mind are as listed above. I don't like the added disassembly complications and I wish the striker safety didn't protrude out the top of the gun so that RMR cuts could be simpler. I will also note that the slide serrations were not as aggressive as the media made them sound. They work, but the slide finish is a bit slick (not as slick as the older Gen 3 Glock finish but similar) which made them feel pretty tame. I don't think they'll shred any kydex.
CZ P10c: Best trigger of the bunch. Most aggressive grip, but not too aggressive. Very low bore axis also. Has what I consider to be a perfect sight picture, just the right amount of air between the sights, however they are that silly glow in the dark type. The trigger is not as crisp as a PPQ in my opinion, in spite of what many YouTubers claim. It's in the same ball park as the VP9 for trigger, although I do prefer the P10c trigger to a VP9 trigger personally. The grip angle is similar to a Glock or a 1911 with a round mainspring housing, but the gun does not have a grip jump so it feels nicer than a Glock to me. The Mag release button is indeed stiff. The stiffest I have ever felt on a pistol, actually. Pressing it for the first time, it wasn't moving and felt like it was jammed in a stuck position until I added enough thumb pressure that it budged just a tiny bit and the mag popped right out. It's an odd Mag release, but it's not bad. It would be easy to get used to and I think it's a good design since I am often concerned with ambi Mag buttons that I could bump the outside button and accidentally drop the Mag while carrying. I think the different design of this release will help negate this possibility, although I personally still prefer a single, swappable button so that the possibility isn't even there. That's just me.
FN 509: This one was the one I actually expected to like the most, but it is not. It's a nice pistol, but I like the other 2 more. It is indeed modular! The trigger group is the serialized part. I haven't taken one apart, but my go to guy at the shop I frequent field stripped it with me. It looks like there's a roll pin in the rear that needs to come out to change frames, so still not the easiest to swap, but not bad. The trigger is average. It feels like an M&P "1.5" trigger. It's not as nice as the 2.0 but more tactile than the original M&P. It's pretty gritty when new. Will probably smooth out nicely, but I don't think many will be raving about the trigger. The grip feels good, nicely textured, not as aggressive as the P10c and ever so slightly more aggressive feeling than the APX. I didn't take that close a look comparing bore axis of these guns but the 509 felt like it sat just a bit higher than the other 2.
If anyone is curious how I rank them, just go in order of how I listed them in this post. In spite of my complaints on tiny aspects of the APX, it is actually my favorite of these 3 new guns. I like the grip, I like the trigger, I like that it's modular. If Beretta ever releases one that addresses my RMR concern, I may just go ahead and get one and overlook my other nitpicks. I don't actually own any guns with RMRs on them or plan to carry with one, but I don't like the idea of not being able to have the option in the future (or if being stuck with a proprietary Beretta one if that's their plan).
Edit: Just to clarify my flip flop on claiming I liked the P10c the most last night, I did when I first held it. After going over pros and cons in my head, I decided that I don't see much point in the P10c over other guns. If I want a CZ, I may as well get a more classic one. Newer modular guns seemed to be a gimmick to me at first, but I think in the future I'll be trying to stick with them. The most common major failure on a polymer gun seems to be frame cracks so if I ever have a problem it's nice to know I don't have to send the whole gun in for warranty service and pay a transfer fee to get it back from an FFL when I can just buy a frame and change it. Plus there's no more need to buy 2 guns just because I can't decide what color I like. The P10c doesn't have this advantage. The P320 and APX both do.