Well I think DB would do more damage and has the combos to mix things up. I also find the animations and sound effects way cooler than Kat. That said, if you really want to have capped melee/lethal defense you should go for Kat. Damage should be pretty good too. You could probably make an extremely high defense build since you only have to focus on ranged/aoe defense. You'd be a damage monster with tons of mitigation.
The quick and dirty version is Kat/BS for non-defense sets, and Dark defense based sets for maximum mitigation. I think /fire being more click based would cut into DB more than it would Kat as well. The plus side is you could self PL the DB/fire to 50 easier if you wanted to have a level 50 asap.
My vote would go to katana over dual blades.
On one hand, I don't like the combos from dual blades. Miss one attack and the chain is ruined. Bah.
On the other hand, Divine Avalanche rocks !
But I'm somewhat biased as I plan to roll a katana/regen Brute when Freedom goes live 
My Broadsword/Fire scrapper is fairly tough if he plays it right, the +Def from Parry goes a long, long way to keeping you alive.
Plus on Brutes you can slot it for Def and let Fury buff the damage, so the DPS hit isn't as bad as it can be on Scrappers.
Dual Blades doesn't do much in terms of survivability at all, plus it dislikes clicky secondaries (Fire is fairly clicky). DB/Fire won't be bad (you'll be able to PL it out of the low levels pretty easily using the lowbie farm) but it will be squishy.
(ie what Amy said)
alright, think i'll wait for the katana then. wasn't thinking about the click heavy aspect of fire, and your right, that would make DB a bit more of a hassle.
thanks for the tips everyone
Title about says it all. Wanting to make a fire armor brute, but can't decide if I should go with Dual blades or wait for katana. It's a concept character, so it's only between those two, but they are pretty much the only weapons sets I have no experience with.
Anyone have any thoughts on one being better than the other, or is there even a notable difference?