Speed's a big deal in this game, really. Unless you're playing against really good people, you'll stay alive much, much longer in a fast mech with just a little armor than a big, heavy mech that only goes 120, but has ungodly durability. Not to mention that there's not an awful many weapons that can top the standard 900-pen. rifle. They might have however much more punch, but when you factor in the reload time of the weapon it's usually quite a bit weaker. I could put a few 1600-pen. cannons on my mech, but I'd only be able to shoot them every 5 seconds or so, and they'd be difficult to aim.
1600 x2 = 3200.
900 x4 = 3600.
So you get more damage per shot, a much higher RoF, better accuracy, and more / cheaper ammo. Even if you got three or four of that same cannon on without sacrificing too much speed, the rifles would almost always be a better choice.
By the way, good call on picking just one weapon for your mech and sticking with it. I can't tell you how many people I've seen put a dozen different weapons on their machine, then just use the one or two. Now, having a backup weapon is all good and well, but why put on two or three, and decrease your speed by so much, when you could just put a couple 125lb plates of armor on your normal weapons, so they wouldn't get broken in the first place.