I'd have to say both. When it comes to comparing both the Beam Rifle and the Sniper Rifle, there isn't much difference. What I will say is that the Beam Rifle DOES have more range, and you're able to fire more shots, as long as you don't fire too quickly you shouldn't need to reload. With the Sniper Rifle, really it packs the same punch, but you maybe better off with its ridicule (You may prefer it, depends on you, really), and you can fire 4 shots without it over-heating, which is more convenient in battle as over-heating with the Beam Rifle takes time to cool down. There isn't much to compare, as both do the same damage, it's really down to the skill of the player. But, the Beam Rifle as I said does have the advantage range wise but also I think it slightly more accurate, especially since the ridicule is smaller.
But, the experience with either sniper can differ, as it seems to give off a different feeling when you look at the Halo games both snipers appear in, and also depending on which Halo game you're playing can make you feel different towards via whatever sniper you're using.