I have been comparing these two laptops and have found the memory and cpu to be about on-par with one another. The only remaining factor (which in my case is a relatively big factor) is the graphics capability.
Playing games that range in required graphics resources from League of Legends to Planetside 2, I want a flexible machine.
The major factor that I am curious about is how much Windows 8 will slow down the Lenovo with its dual-SLI cards versus the MSI running Windows 7.
The difference of gaming between Windows 7 and Windows 8 may be 0.5-2fps in it at max difference majority are in favour of windows 8 for gaming than 7. And the 750M SLI vs 740M for reference ill use Elder Scrolls: Skyrim as an example the 740M on ultra settings managed average fps of only 14 while the 750m averaged 58.9fps a big difference when turning the settings up. I'd say opt for the 750M SLI especially in a game like planetside 2.
Game performance should be roughly equal when comparing Win 7 vs. Win 8. There was a article from a different site awhile ago which concluded that overall performance was marginally better in Win 8. Benchmarks of Battlefield 4 have shown better performance in Win 8 compared to Win 7 by a decent amount depending on the CPU used in a desktop. At 1920x1080 I believe the performance increase with a GTX 770 or Radeon HD 7970 improved by 3 FPS to 13 FPS depending on the CPU in Win 8 vs Win 7.
For a single GT 750m in Win 8 to equal a GT 740m in Win 7, Win 8 would probably have to have at least a 15%-20% performance penalty compared to Win 7. For dual GT 750m the performance penalty will probably have to be at least 30%.