Depends on a few things:
1) The cost. A Plasmagun is 15pts, 2 Pistols is 30pts. I'd go for the gun every time.
2) Whether I actually want said unit in combat. Most of the things I can give plasmaguns to are also things that I wouldn't want in combat. If I did want them in combat (or wanted to leave the possibility open), I'd probably opt for meltaguns (or other assault weapons) instead.
3) What is the rest of my squad using? e.g. there's no point having a sergeant or character with 2 pistols if the rest of the squad wants to be firing at 24".
4) What am I sacrificing to have 2 pistols, and can my character actually do anything with the extra attacks? To elaborate, if I'm using 2 pistols, then I'm usually doing this instead of upgrading a
CCW. And, if I want the unit to be in combat, wouldn't I rather have a Power Fist, Power Axe or some other weapon? One possible exception is the
DE Haemonculus - since he can have 2 pistols, and still have an upgraded
CCW.
5) Are the pistols my character/sergeant has access to actually useful? I doubt anyone will be intimidated by someone dual-wielding las- or bolt-pistols. And, whilst plasma pistols are scarier, they're also very expensive. To put it another way, are these pistols actually adding any useful firepower to the squad?
6) These may, of course, be overridden by the Rule of Cool.
