I like the large backstraps on the gen 4 and 5 Glocks. Takes away just about any chance of slide bite. Really makes it feel good in my hand, but I still prefer a springfield
I’ve got a Gen 3 model 19 and a model 43.
I find the 19 to be too bulky and heavy for carry but I really like the 43.
I do carry the 19 in my truck on trips because of the capacity.
About 20 years ago I decided to see what all the Glock hype was about and bought a G17. At least once or twice a mag the ejected brass would come straight back and hit me in the forehead. Didn't like getting "shot" by my own gun so I sold it and never bought another Glock. I wonder if they fixed that in the newer generations, I don't know, way too many other pistol manufacturers to choose from and so far never had another manufacture pistol that did that.
To add to the recommendations above: when you pick your caliber, don't do it based on expected recoil or muzzle-flip. I've found 45acp in a Glock shoots "softer" than 9mm... weird, I know.
1) Is it for CCW? Then narrow down selections based on pistol size, for where you carry on your body, and your body type.
2) Do you want large capacity on-board, or available in a reload? Will probably determine single vs double stack mag models...
3) Make a checklist of the different variants and go try them in your hand at a store, range, your buddy's "armory"...
4) Try to test fire your top three.
With modern ammo, and considering the duration of most "gunfights", you could make a case for many calibers and capacities. It really depends on what you can carry and shoot well.
Gen 3 grips "feel" larger than Gen 4. I have no experience with Gen 5.
Standard, Compact, and Sub-Compact all have different feels (the backstrap hump, finger grooves or not, etc).
Don't forget that you can easily modify the grip, if that becomes you're only sticking point.
Some of us who are right handed and left eye dominate will get the occasional forehead hit. Some have said that cutting down the ejection port can eliminate that, the theory being that it hits the edge of the port, and directs the case upward. I haven’t tried it. Also, tuning the ejector upwards slightly might help, or it might make the problem worse, if the previous theory is correct. If that is the case, tuning it down might help. Maybe a slight right cant to the gun would eliminate it. This only happens occasionally to me, and I can’t say at this point if it is certain guns, or all or them, as I have a dozen of them, and it only happens a few times a year out of thousands of shots fired. If you have problems frequently, send it back to Glock. They fix problems for free unless you caused the issue with some permanent gun mods. I know you sold yours, but this is for others reading this.
I have Glocks and have never had 1 bad issue, all were great and I still own a couple. I believe it is one of the great and reliable handguns on the market. 19 and 26 theses days with the 26 and 15 rd mags as my primary.