DB/???
Survivability? Probably Dark, which is what my DB brute is, followed by Electric, then Fire.
Any of the 3 will be a crapton of fun though.
I don't think I've ever heard anyone suggest that Fire would win the survivability contest. Not because it's junk; all three sets you named are resist-based sets with generally good numbers, although each is stronger in different areas. But Fire's additional tools are about damage, not direct survivability improvements.
Dark and Elec, on the other hand, both have some control abilities, with Dark actually having a couple different options. Elec's endurance drain probably won't completely lock down much of anything, but should significantly reduce the number of attacks even Elite Bosses manage to make. Dark, on the other hand, has an actual hard control, but it generally won't affect anything but minions.
All three have self-heals with different advantages and disadvantages, though Dark's is probably one of the best.
Dark is generally known as a high-endurance set. Elec isn't any cheaper for the early part of it's career, but it gets advantages later on... including an endurance recovery power on a very short recharge. So if you like the idea of never having to stop to rest, I'd say Elec has the edge there. (That's actually part of the concept of my DB/Elec in fact, and part of what made me pick Elec over Dark.)
/fire does more damage. It will be the squishiest of the three. One other knock against it is it will cause the most redraw of the three. The fun factor will be high just because you are going to pump out a ton of damage. I only have a couple /fire brutes that I used to learn about self PLing. One should hit 50 tomorrow and then I'll work on the other that's at 40. Minimal experience outside of farms, so there is little I can add. I am aiming to make an alternate builds so they can be played for general content. From my play experience as a support-o-holic, I wouldn't put too much into forming an opinion of FA until it's got IOs in it. FA on SOs is...lacking in my book.
/elec is more end friendly. My only issue with it is that brutes get Energize at 28 versus where scrappers get it at 20. Elec is working it's way towards my favorite armor set. I do have a level 32 DB/elec and level 31 Spines/elec scrappers to whom I have enjoyed. My Dark/elec brute was actually set up for deletion, but I talked to a SG mate and I was willing to grin and bear it until I got Energize. It looked so good on paper and yet I hated it so much I repeatedly put it on the self. Now, it's becoming one of my favorite builds. I got a Elec/dark Hardcore tank, and I'm rolling a SS/elec brute somewhere. I did say I liked ElA.
/dark is sturdier. It has a bunch of tools that work with teams as well. It is, an end sucking behemoth in the making if you let it be. Outside of getting KB protection, you will want the +end proc in DR asap. Highest DA I have is a level 40 Dark/stone tank and I'm happy as hell to be close to epics and then getting Cardiac once I Incarnate. I basically have a SO type build outside of the +end proc and it's a pre i19 build so I don't have room for Tough/Weave, but I can't imagine running all of those toggles on an SO build. I'm not saying it's a bad set. I just can't imagine rolling one without the intention of IOing it out and getting it Incarnated. Same can be said for FA mind you. I got a Claws/dark brute, but it's around 25 and I haven't really slotted it out yet for some reason. I need to do that and then take it for a spin. I'm just glad Claws is way more end friendly than /stone.
Something besides fire. Not because its not good, but because I think of Burn. Burn is just too good not to use, yet in my opinion breaks the flow of DB's too much. Between Healing Flames and Burn, I think you lose out on the strength of DB. Everyone has their opinions how to play things, but for me, I like the second two combo's for general play, and I think Fire would break that flow. I have a DB/Regen, and all the Regens clicks just took the fun out at the end, even though he was a great ride to 50. Fire, will be nearly as clicky, though more offensive, still disrupting.
I think I would go Electric. Recovery, increased recharge, a heal. Works nice against the end game content. When I play /fire, I tend to play a primary that wont get in Burns way, when I play DB, I play a secondary that doesnt get in its way.
GL
The best pairing for Dual Blades is Willpower. As TrainRobber somewhat pointed out, using your secondary's powers can lead to re-draw delay. Willpower is the only set that is completely hands-free because all but the tier9 power are either toggles or passives.
Okay, so let me ask. I'm thinking of building a DB brute. I just haven't figured out if I should go Fire, Elec, or Dark with this toon? The concept with this toon would work with any one of those. I really don't know the secondary's too well to make an educated choice. Which has the most survivability? Which is the most fun in your opinion.