I have one of each (except my 27" runs at 60Hz). The difference is bigger than you'd think. That would be an argument in favor of the 30" monitor.
On the other hand, 1440p is usually much less expensive.
If you game a lot, go for the 27". The higher refresh rate would help. The GTX 780 Ti is good enough to get you over 60 fps in some games, even at 1440p.
Edit: I missed the "sli" part. Obviously gaming is very important to you, and a pair of 780 Tis will definitely get over 60 fps in anything. Another vote for the 120 Hz monitor.