From what i hear, Blood is better threat on boss fights, where Frost is better for mobs. Frost though has less damage(2% from imp frost presence), and blood is more durable(getting 2% of the damage they do as heals from imp blood presence if they spec into that). Frost has better AoE with Howling Blast, glyphing it gives frost fever to everyone that it hits, along with blood boil and Death and Decay, you most likely wont lose threat as frost(considering me at t9 and i was holding threat with a hunter that had death bringers will(i think is what it is called, it the one drop from loot ship 25 man) and GS on wow heros was most likely 3.3k without getting misdirected to). Blood has alot more ST threat gen, so if your frost, you wont gen as much as blood, but really I never lose threat ST(mainly because i can Rune strike every 2 seconds with my 2h sword since my Tank spec uses Imp icy talons, and i really make it work, everyone but 2 people say i speced wrong, saying that icy talons was a waste, but it goes really well with the frost sigil and also i am as good as a shamans wind fury totem).
As a general rule of thumb though, Frost dks have more migration on damage(3% melee dodge and 2% less damage), and blood is more i want to say durable, but its not quite the right word, in terms of taking damage, but Blood has worse AoE threat gen(only one being Death and decay and boil blood, not sure about heart strike, never went blood, i never liked it lol), where as Frost has a huge AoE threat gen(i crit about 6k on Howling blast and since killing machine is up 90% of the time i use it, i almost always crit) boil blood does more damage if you glyph howling blast, and Death and Decay is very good threat gen, so its your choice. I prefer frost though, considering with Imp icy talons and 2h's i am hitting hard enough to give me alot of ST threat gen with Rune strike up every 1-5 seconds.
well we are about the same spec as u and im a little better geared and honestly i think i aggre with u and im going to be fairly sad when blood is the only tank spec in cataclysm . and i thank you for replying cause its still going back and forth between blood and frost on my realm and since i havnt played in 2 months i figured ide do a little research for when i start up again in 2 weeks. haha