Well the usual question is...
Do you want easy multimonitor support or not?
ATI is just better for that.
Otherwise...
Mobile Graphics Cards - Benchmark List - Notebookcheck.net Tech NVIDIA GeForce GT 540M - Notebookcheck.net Tech AMD Radeon HD 7670M - Notebookcheck.net Tech
You can't exactly go by their standings in that list since the cpu in the test systems isn't identical. Vantage scores higher in the 7670 average. 3D06 scores higher for the 540.
But they're so close in performance I wouldn't say either way. I mean, if you believe the list, the AMD edges out Nvidia. But if you like being able to 'dock up' and watch/play on 3 monitors, (and the lappy allows for that) but the ATI cards often give eyefinity support (didn't confirm it's on the 7670 but I think it should be, should check if that's a benefit to you) and it's something I like myself, so would go with a laptop that supports eyefinity output. 3 monitor support is nice.
As always when they're so close, it's the little things that make more difference than the raw gaming power, since neither is far and away a clear 'winner'.
Gray