No Advantage

Going through the 9mm's GURPS stats and there's some fun stuff going on from a player's perspective that might not pass a reality check, but the system doesn't go fine enough to catch.

A Glock 17 and Glock 45 are the same weight (2/0.6), and have the same number of shots(17+1(3)).  They have the same bulk.

But the 17 does more damage (2d+2 pi vs 2d+1 pi) and has longer range (160/1,800 vs 140/1,600).

An M&P 9 does the same damage as the Glock 17 and has the same number of shots, but it's heavier in all respects (2.2/0.7).

The hybrid Sigma appears to have some advantage.  The drop in damage and range is at least accompanied by a lower weight, (1.9/0.6) but also loses one shot (16+1(3)).

The Wonder Nines™®© don't compare favorable once plastic fantastics appear.

The S&W 59 has the lower damage and range, 14+1(3) shots and is heavy at 2.3/0.6.  TL8 magazines fix the capacity at no weight penalty, leaving you with a heavy Glock 45!

Beretta's M9 has the higher damage and range, but fewer shots (15+1(3)) and more weight (2.4/0.6).  Like the 59, there are 17 round magazines commonly available at TL8, but have a slight change in weight to 2.5/0.7.

The ur 9mm double stack, the Browning HP gets the higher damage, lowest capacity and has all the mass.  13+1(3) shots and 2.4/0.5 weight.  TL8 magazines up the capacity to 15+1(3) with no weight penalty.

None of these stats account for ephemeral things like the HP with Spegel grips fitting my hand as if the gun was made for it.  There's rules for that (Weapon Bond perk; High-Tech p.250), but it's not on the weapons table and it requires the player to do more reading than they typically do.

It doesn't account for me liking Glocks better than M&P's, or Marv and Harvey feeling the opposite.

I am often reminded of a scene from Farscape where John Crichton is comparing two pulse pistols back and forth.  They appear to be identical, but only one is Winona.  Winona fits him, the other doesn't.