Nice collection! My one big gap in my collection is a good cartridge SA revolver (I do have two blackpowder revolvers, but no cartridge guns). I seriously need one or two, and all the more so now.
As for .44spl, .44mag, and .45Colt, the only one I have in that group is the .45LC. It is a more versatile caliber (though I suppose the .44mag is a more versatile chambering since it can take both .44mag and .44spl). It can do anything the .44spl can and be downright gentle to the shooter, but in the right loads (and guns) it can do 80-90% of what the .44mag can and pretty much anything that most shooters will need it to do.
That said, I do need to eventually get a .44spl. Probably one of the L-frame S&W's if I can ever find one at a reasonable price, or the Taurus 445 or 431 (nice CCW sized big bore revolvers).
I may eventually add a .44mag, though I think I need to replace the .41mag S&W that I sold a few years back and desperately miss before getting a .44mag. It will do most of what a .44mag will but without the punishing recoil of .44mag or a stout .45LC (at least, I experience the recoil as a much lighter recoil impulse). Still, I may get the .44 first for the same reason I sold the .41 to begin with (ammo and reloading components cost and availability).