For a 'ninja' kit I prefer the MP5, more accurate than the P90 and quieter. That with Bomb Squad, UAV jammer and Dead Silence and it's good times after breaking enemy lines.
The Barrett is pretty garbage in this game, no where near enough damage to justify it's poor accuracy, M40A3 is the duck's nuts of the sniper rifles.
P90 is definately a good CQB assault weapon, a little short range for some maps.