If the radeon hd 7990 was $150/£100 cheaper than the gtx 780 ti which would you get

Personally i would still choose the 780ti as it is more reliable performance from game to game, as well as will consume MUCH less power.
the performance difference between the 7990 and the 780ti is also not huge either, which makes the single card solution much more favourable in most cases.

With the 7990 you will need a better psu than you will with the 780ti, as well as need to deal with crossfire drivers and crossfire profiles for games that may not always work perfectly like you would with a single card (i would choose even a single 290x over the 7990 mind you)