I chose Rate of Fire even though that only applies for my pistol choosing. (Unless I am able to obtain a Rex).
For my main weapon, I think it all depends on a number of factors, just the best combination of them. Like, what good does damage do if it has no range and stuff. The MP5 was an ideal gun for me since I could kill someone at so and so range with like 3-5 bursts. The UMP however, at the same range, would take me noticeably more bursts, such as 4-7.