Probably the VX6-7 or the serpent. The serpent has a slightly higher ROF and less recoil but the VX has a shorter reload. The high ROF weapons are almost universally considered better in CQ, and the damage falloff is no worse than the stock solstice, so mid range should be fine.
Personally, I'm looking for the best mid-to-long carbine. I can do well close up with any of them, but past 60 meters..... (Yes, LA shouldn't be engaging that far away, but it happens when defending small bases)
Honestly, just stick with the Solstice or the Solstice SF. They seem to be just what you're looking for.
For VS the only carbine that comes close is our "NC style" carbine the pulsar c. Like NC weapons you can get the adv forward grip, compensator if you prefer and High velocity ammo if you prefer. You can't snipe with it like the NC high damage carbine, and at 100m you do about 30 less damager per shot than their high damage carbine, but it's the closest carbine for VS.
What everyone else is saying. VX6-7, and get the extra 200 certs for a adv. laser. LevelCap's Review: