Mw3 guns advice on which route to go?

ok so i love the mp7 and the msr soooo much, by far my fav weapons ingame, and i want to focus on 1 gun purely so which would take me farther if i got as pro as can be with one of these weapons which would take me farther as far as kdr, im amazing at the mp7, right now im using attatchments rapid fire and extended mags , hipfire i beat even fmgs and midrange is nice but longrange sucks, msr im ok at quickscoping and good at hardscoping, so focusing one of these guns and being the most pro i can be which would take me farther and make me do better in matches, also when using overkill and using p90 as ur 2nd primary whatts the best profficiencies and attachments to use on it?