In one way, I'd say there's not a spit's worth of difference. Both can solo at least to b-grade with little problem.
Daggers party and exp fairly well---the BD's (palus knight after class change) party support dances are usually well appreciated as well (even through level 55).
Don't know much about the assassin--but the palus knight was easy to play and leveled well and had a class change quest that was pretty simple.
I don't think you can go far wrong with either choice. If you think more pvp...maybe the dagger has a serious edge solo. Getting through the months of PvE will be enjoyable if you play BD--at least you'll have friends :)
Don't forget about Shilen Knight. They are tanks, though more attack oriented and as DEs have lowest Constitution, some people doubt about their tanking ability.
They are not easy to solo though ... completely solo, without buffer behind their back.
If you ask me go archer, they are the owners on the end game. Shilen Knight (Palus Knight is the light elf knight) is a great mage tanker, it's cubics are nasty but you're still being a tank, and you will need an buffer once sk is the only tank that don't have a self buff to add m def.