I prefer .357 mag any day of the week over .44.
Tend to agree with the OP, .44 mag is fierce, plus with a nice .357 such as the beloved 686 you can always use .38 special for practice. Let's face it magnum rounds ain't for punching paper!!
And the Glock 17, what a trigger! Especially on the Gen 4 - that thing resets so quickly you can almost double it. There's a semi undocumented feature on the trigger, I was taught at a range in the US, to achieve an even shorter/quicker reset time.
As far as the 454 & 500 rounds go, nah - not for me, way too much of a handful. I suspect you would need a very specific application for that type of weapon. 9mm is just so sweet, especially in something heavy like the 92FS or the updated 92A1 the recoil is very manageable indeed.
Co2 is fun, don't get me wrong - but they're a very long way away from the experience of a cartridge pistol. Sure you can learn the fundamentals, but you can't teach yourself recoil management, proper grip and reset time.
Just under a month to go for me - get reacquainted with a Glock 17.