That's really a hard choice, but go with what design looks the best, or what fits your "character" better. Personally Im working on a mando heavy weapons guy, the backpack design i have is a pain but in the end it will hold my chaingun and sniper rifle on it... It's seriously going to be heavy
Do you have any design ideas you could show us? Draw something up for us to see. As mentioned earlier, go with what fits your character. If you're all stealthy, you may want the sniper. If you're all out fighting in the front lines, go with something better for that.
honestly, I suggest that you do what you want to do. Do you play any first person shooter games? If so, which do you prefer on those games? Do you do any shooting for real? If so, do you prefer to shoot close targets or long range targets? I mean honestly... It's up to you. If you're still on the fence... you may consider something like the SAMR/DMR that the US Army/US Marines are using now. It's a standard issue assault rifle with optics attached and a little fine tuning. Gives them the ability to have an "Advanced/Designated Marksman" within each rifle squad. Also, You may check out my WIP thread and look at the carbine my character has, I'm considering mass producing them for distribution.