Both are Mid-Range graphics card. 8850M is built to the GCN architecture which is same that's working in the consoles. Whereas, GT750's overall performance is little bit better than 8850M. It mostly depends on what type of games u will be playing. Bcoz both the cards will be able to handle, recent games low-to-med settings at 1080p.If you're comfortable with overclocking your vBIOS, you may be able to stretch it a bit more.
My personal choice has always been AMD/ATI Radeon over Nvidia.
But the overall processor speed of the lenovo's is great than the dell.And moreover dell is having dual core and lenovo has quad core.So i decided to get it.So GT 750 is best than 8850m or not.Just simple answer