You don't want player choice then. You have it and you don't like it. You want one gun with different skins.
I edited my post to make things clearer: (It all depends on how you define player choice, I prefer player choice in terms of allowing them to do more in the gameplay, rather than picking what starting weapon they have but having very limited rock paper scissors gameplay. Lots of different weapons in the same class doesnt lead to great player choice, one versatile weapon in each class would.)
Slightly different streams of argument, whats good for the campaign sandbox is hardly going to be good for the multiplayer sandbox. That said my reason for wanting one gun to rule them all is all about player choice, I simply mean in terms of what they can do within gameplay itself rather than by what weapon they choose.
I couldn't really speak loud last night, and Xand had a similar sounding but pretty different argument to me, which he kept hijacking my sentences to talk about.

but yeah, in multiplayer I want one very versatile gun to rule them all, I want a gun that has good range, can be used in a variety of situations, and really opens up the battles to player freedom, a gun which good players can use well and creatively to out gun other players less skilled with that weapon. I dont want to be restricted because I chose a BR but the other dude chose a DMR, I want the gameplay to focus on player ability, not some meta rock paper scissors game decided in the main menu.
So yeah, having one versatile weapon doesnt limit player choice, it enhances it, because when it comes to gameplay theres a much larger subset of possible outcomes, than if we get a very rock paper scissors style game with pre-determined outcomes.
I mean sure, you can argue that player choice is all about letting the player choosing the gun they like more, and I understand why people would want a range of different weapons, but as far as im concerned thats all just aesthetic fluff, I dont care what the weapon looks/ sounds like, as long as the gameplay is good. That if all the weapons play the same, if they dont well then that just wouldn't play like Halo anyway.
DMR vs BR is going to radically change the way the game plays based on whichever one you choose, it wont be like a binary choice between the DMR and Needle Rifle or Halo 3 Carbine and BR, the way battles and players use the gun is going to be very different based on the fact they have pretty different base gameplay mechanics behind them. If the DMR is the gun that rules them all, but that means very spammy bloomtastic gameplay, I would rather not use it lol.
So really theres two arguments going on there:
1. Weapon versatility (I want the starting weapon to have good enough attributes to really open the floor to a skilled player to make interesting use of that gun.)
And
2. Base gameplay mechanics and how combat plays out, (I dont want to use the DMR with bloom because I dont like that style of base gameplay)