Wow, I'm late to this party, but gonna chime in in response to the OP anyway...
I have a Gen 3 and a Gen 5. My issued duty gun is a Gen 2, and the gun I usually qualify with is a Gen 4.
The grip texture on the Gens 4 and 5 is WAY better than the previous versions.
As for triggers, some say there is no difference, but I do notice a difference between the Gen 4 and Gen 5 trigger feel. I prefer the Gen 5. It's still "mushy" like a Glock, but somehow it's also more tactile/crisp/positive/whatever... at least in my experience--especially on reset.
Using the same sights (Trijicon Bright n' Tough) on my 19 (Gen 5) as on my 17 (Gen 3) I can shoot tighter groups with the stubbier Gen 5... take that for whatever its worth. I did get to compare my Gen 5 19 against one of the Gen 4 17s at work (standard Glock steel sights). Still shot better with the Gen 5 19 (knocked down 8" plates faster and with fewer misses--could have been the due to the sights). Again, anecdotal, so take it with a grain of salt.
In my opinion, if you're buying a model you don't already have, get the Gen 5, if available. If you already have a Gen 3 or 4 of a particular model, only update if you have cash burning holes in your pockets.
I don't have a lot of experience with various brands of pistols, but I can say this. Shot my FIL's SIG 229 (or whatever the SEALs used to carry... 226?), and the trigger was a huge disappointment. Now, to be fair, his pistol is California'd, so it may be some kind of compliance thing, but there was literally NO reset on the trigger. When you let off looking for a reset, you just get a limp trigger (no spring force "assisting" your trigger finger back). Once you let ALL the way off the trigger, it kinda "falls" into place and you can shoot again... I really didn't know what to think... I was really disappointed. Also, the high bore axis on the SIG, relative to Glock, really did feel outdated to me... maybe it's the way I've been trained to grip a pistol, but it did feel top heavy. When I look at other brands of pistols (CZs being the exception), I see this high bore axis and it puts me off... but I'm just one guy on the internet with a bunch of opinions.