There have seen lots of tests which have concluded that in most games, a pair of 7970's have outperformed a single Titan, from anywhere to a few FPS, to double digit figures. There are several reasons why I would get the CF 7970's (specifically the GHz editions):
1. There is physically more power to muscle through games.
2. A wider variety of coolers, brands, and accessories is available for the 7970 cards.
3. Different companies make a far wider variety of water cooling parts for the Radeon cards.
4. It costs less unless you get the absolute high end cards.
There however are several reasons why you could want to get a single Titan GPU:
1. It consumes less power than CF 7970's
2. It could be compatible with smaller boards (M-ATX, ITX) where you only have a single 16X slot for use
3. It could
produce less noise, but it depends on what kind of 7970's you are comparing it too
4. Just for the wow factor of being able to tell someone you have "The Titan"
As for what you personally want, it's completely up to you in the end. I'd personally want the CF 7970's for the brute performance, the looks, and the wider variety of water cooling and general accessories available for it, plus there are many more brands for it. Asus, MSI, Gigabyte, Sapphire, RARELY XFX depending on the card, and HIS all make solid cards. I wouldn't always say that about XFX, but you have to be careful about which card you get.
Good luck, I wanna see this system after it gets whichever GPU!!!