I am not quite sure, since I can't find any benchmarks for GT 60 Dominator 424 but I would suggest going for GT60.
There are couple of reasons really.
(a) 8xx mobility video cards, from what I've heard, are going to be able to boost battery life.
(b) SLI is good, but some games don't support 2 video cards therefor the performance itself will be substantially lower than it should.
(c) 870m is a promising high end mobility video card, it has nice specs from what I've checked.
I'd go for MSI GT60 Dominator 424.