Does this mean I should go for heavier bullet rather than higher muzzle velocity? Or should I simply look for something that has high fragmentation on contact?
Hard to say, now. In the past, most would say “use whatever _your_ gun ‘likes’ to shoot.”
Here is how that was explained to me: every gun shoots every brand(and this includes different bullet weights) of cartridge differently from every other gun. IOW, what works well in _your_ Glock 20 may not shoot as well as in _my_ Glock 20. Therefore, one should try shooting as many different loads as is possible then select that load which shoots ‘best’-in your gun.
Considering the run on ammo that’s been going on lately, this may not be feasible. (Yes, even I have succumbed and recently purchased another thousand rounds! You might give SGammo.com a try.)
I personally carry a Glock 23 and it just seems to shoot 180 grain bullets more accurately than any other weight. Doesn’t seem to matter if they are hollow-points or FMJ, it puts ‘em right where I want ‘em. Generally the lighter weight bullets have higher velocity(but lesser foot pounds of energy) than the heavier bullets.
By “fragmentation” I assume you are referring to the Magsafe-type of frag bullets? I have no experience with those whatsoever. You may try asking your questions on one of the many Glock forums.