If you're looking for longevity, the GT60 from MSI would have my vote. It's a big laptop, but very solidly built. The Lenovo is built well too, but the Dominator line from MSI is very tough. I would also tend to think that a single GPU (the 870M in the GT60) would be a better long term solution than SLI.
The y510p with SLI 755 will run better in games that support SLI, and is quite thinner overall. The GT60 will give better performance in games that don't support SLI. You might look into if the games you play support SLI.
The Y510p is the one with dual gpus and without a disk drive. (6th model on the Lenevo site)
I was also wondering what laptop would last the longest, or if they are both pretty sturdy. I think that the amount of time it will last is the deciding factor for me.
If you're looking for longevity, the GT60 from MSI would have my vote. It's a big laptop, but very solidly built. The Lenovo is built well too, but the Dominator line from MSI is very tough. I would also tend to think that a single GPU (the 870M in the GT60) would be a better long term solution than SLI.