Well, they both have their advantages and disadvantages
i.e.
The beam rifle does not leave a smoke trail (but the beam can be seen)
The sniper rifle has (relatively) infinite ammo (keep getting ammo)
The beam rifle does shoot faster (but does overheat)
The sniper rifle makes a noise when zooming in (so even if they're not screen cheating, they'll know you're about to shoot someone)(beam rifle does not)
etc.
Everyone who has played more than a few times knows of these. Just figured I'd point them out. I personally, if I had a choice, would pick the beam rifle for multiplayer and the sniper rifle for single.