It always confuses me when people say something like this. It's like, having a second character is taboo. I'm not saying it's what you're doing, but sometimes I forget that some people only play one character, but at the same time, it's weird when they're surprised when some people play more.
It really depends on what you're doing. For leveling, nothing beats Blood. Frost works a lot like ret pallies, in that a lot of your dps comes from random procs. It makes it really hard to kill stuff by yourself when those things don't proc. I'd rather go with something more consistent, and in this case, has more survivability and self-healing.
Frost:
An icy harbinger of doom, channeling runic power and delivering vicious weapon strikes.
Unholy:
A master of death and decay, spreading infection and controlling undead minions to do his bidding.
Why split the credit with your pet when you can take it all for yourself?