you all have completely nailed it.
my kj-based hicapa build has all 1 handed controls (which cost a pretty penny), and my single stack is a pain in the ass and doesn't get through a short-loaded long mag. I have a knockoff hogue grip on my single stack, and it's the only one I've found comfortable, but at which point I might as well be using a double stack, it's so big. (and it completely nullifies the point of a single stack). I've seen and handled some of the 4-digit steel custom/high end jobs and sorta gone "meh".
glocks are only ok for me. having spent over 4 digits on my TM 17 build, I can only shrug and go "ehhhh". and even after having touched literally every little fiddly bit, I still have no idea how the hell to reassemble one.
USP's are all sorts of boner-inducing for me. the sights line up SO SO SO DAMN GOOD. points well. sooooooo good looking. and you get to constantly yell "I'M JACK BAUER!". doesn't matter that the grip is uncomfortable and the mag release gets stuck on my middle finger unless I break off the trigger fully. haven't got around to getting one
Sigs are effing brilliant. they only require one upgrade part (the knocker/striker) and you're set for life. the trigger is nothing special, and I'd kill for a slide that says 227/.45 acp on it. but there's a reason I rotate through my other pistols while still always having the Sig on my belt. the 228/229 is a hair short and pinky-pinching on reloads, despite not actually being much smaller, but if it floats your boat.
I want an XDM and/or PX4 hard, but I spent all my damn money on that glock.
as previously stated, m9's are effing large. hop up is a pain to adjust on the kj but otherwise every one I've handled is great. the kwa/ksc ones will drop a round if you release the mag before empty, but if you can live with it, I've only heard praise.
but yes, go hold one. or several. or all. go to a game and tell people this is what you're doing, and they'll probably let you try theirs. only you and your hands can tell which one is best for you.
my brother-in-law's sister's husband (the hell do I refer to my sister's husband's family? white people.) has seriously near-shaq sized hands. my palms are the same size as his, but my fingers are like a whole knuckle shorter. I don't really fit into most gloves that well. point of the story? most comfortable pistol I've ever held is a Sig 2022, but short of a japan-only plastic jobber, there isn't one available. I've handled and fired a TM desert eagle, TM 5-7, KWA mk23, hi-capas, without any problems that I wouldn't be able to overcome (or aftermarket-parts my way out of). M9's give me more problems than those combined, and yet my favourite, comfort-wise, is a pinky-pincher compact 2022.
tl;dr: comfort doesn't make a lick of sense between two different people. try some different things out, and go from there.
(there was a consignment GBBR M4 here in calgary that had the fire selector shaved down to seriously a couple mm protrusion from the body. nobody at 007 could figure out the eff why the guy couldn't live with a regular M4 selector. he said it was uncomforable with gloves. everybody made the stink face.)
too busy tinkering to play.