Khajiit by a land slide(if you're a min/maxer), especially if you wanto melee, increased damage from stealth and 15% crit damage in melee on top of that, all stealth attacks auto crit if i recall correctly so that's a pretty heafty bonus.
Don't take my word for it tho, i'm just a weekend tester but that's what i've gathered.
Bosmer are good too tho, you don't really need to use a bow, the weapon specilization just makes it level faster and is a non factor once you hit level50. I'd look for stats that are harder to come by on gear and from what i understand, the damage from stealth bonus is only avalible to Bosmer and Khajiit.
I sincerelt doubt you'll be "gimped" if you're not Khajiit, carnage is very powerful but it's very powerful for a racial, it won't win fights for you but you'll definently have a slight edge over non Khajiit Nightblades who melee.
Besides, racals will get balanced, going with a race you hate beacuse of a racial that's likely to get nerfed will make you feel like a fool when it does and you'll be stuck with a race that bugs you.
Go Bosmer and you have a race that's still great for Nightblades and you'll be happy with the looks.
But like i said, i'm working on mostly hearsay, the weekend betas and experience with past MMO's but that's my 2 cents.