do not go for the sli option on the y510p, the seconds card tends to overheat due to a bad design. the msi has a stronger graphics card and will keep it temps in check for longer... i do however recommenced the i7 variant since cpu demanding games will be held back by that cpu...what games are you planning to play? the gtx card is much newer that the old gt755m. and it is also faster. the 860m will be able to play newer games at higher settings for better fps compared to the single 755m. but the gtx 860m has 2 different architectures... the Kepler variant which is slower and the Maxwell which is faster... try and find out if the gtx 860m your getting is the Maxwell variant since its more efficient and faster.... if you can , go for the msi with the i7 and Maxwell card. otherwise just but the information you have given me and taking into consideration about the reliability, i recommend the msi.
gt755m review
http
/www.notebookcheck.net/NVIDIA-GeForce-GT-755M.102020.0.html
gtx860m review
http
/www.notebookcheck.net/NVIDIA-GeForce-GTX-860M.107794.0.html
refer to the benchmarks for the difference.