For longevity yeah the Titan and maybe a 2nd one at some point vs. a 2nd 580 now might be better. Also, you'd have a TITAN dude!
I too could afford a couple of Titans but, while I love the tech and desire the card to a degree, that price just puts me off. So much so, having waited for the Titan announcements, I just grabbed a pair of reference GTX 680's for far less than one Titan. I also picked up two lovely Alphacool Copper / Brushed black steel water blocks, and I still
have change from what that Titan would have cost me.
Gaming-wise I generally score higher and I just
game, so that's alright. Additionally, as I was still
under that Titan budget, I got myself a new 256gb SSD too...just to ensure I properly blew my budget you understand.
As I type the first block is on, 2nd card on my bench ready for dismemberment lol.
As an aside, I would have happily stuck with my two GTX 570's if one hadn't developed a fault. I didn't want
to upgrade just yet, but got royally pissed off when I kept getting hard-crashes during games. When I saw a replacement 570 was MORE than a same-brand 670, well, I decided to move on. When in turn I notced that 680 had come down a fair whack, well, I went for it.