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.
The Lenovo y510p w/ sli has overheating problems associated with the gpu. THEY DIDN'T PUT THERMAL PASTE ON THE GPU. Despite the y410p having overheating problems they did not address this problem with the architecture of the y510p. Still, the fact remains that if a company cant be bothered to put thermal paste on the GPU card then clearly they dont care for the customer. Another problem with the architecture is that it is made of metal, a material that conducts heat well, therefore after an hour your palm are boiling hot and the single fan to cool the system blows that 90 degree air onto your mouse and hand.