I would have gone with the 460m personally.
- First, nVidia's driver support has REALLY caught up for laptops. It's now at simultaneous release with desktop drivers. This means we no longer get the "scavenger's delight" that used to plague notebook users. (I have a GTX 260m, and I love it).
- Second, even though the Mobility Radeon HD 5870 has 800 Stream Processing Units, it has two shortfalls. Stream Processing units are VIRTUAL... there is an INCREDIBLE amount of hyperthreading that goes on, if my memory serves me correctly it's something on the order of 5 stream units = one actual core... so in actuality it's 160 physical cores and 800 threads. The GTX 460m is 192 physical cores for processing.
- Third, and perhaps most important, the bandwidth of the HD5870 is 128-bit, akin to the desktop 5750; whereas the 192-bit bandwidth of the GTX 460m is analogous to the desktop 460m (192-bit variation), but with 2x the GDDR5 RAM of the desktop variation (768MB v. 1,536MB). extra bits mean more bandwidth, and an overall better gaming experience to be had. I wish it was 256 bit (the GTX 260m - the 460m's predecessor - was 256-bit... but nVidia is banking on more cores and GDDR5 being able to offset bit-based bandwidth).
However, in terms of memory bandwidth, the HD5870 has a slight edge (64GB / sec v. 60GB / sec) and a lower TDP (50w v. 75w). Thus if you're looking for enhanced battery life, the ATi may be the way to go. However, ATi's driver support is spotty at best, and to me, I'd rather be a little slower and reliable than fast and have to hope and pray the new driver isn't going to kill my computer.