Please read the whole information about the RTM version of 2008 including Hyper-V in the TechNet Magazine.
Offcourse this information is bias, but it does give the impression performance should be better with Hyper-V then with ESX, but features like VMotion aren't available yet.
The other disadvantage of Hyper-V is that the hardware is configured in the Virtual Machines again. This does seem to increase performance, but you lose the advantage of being able to move your VM between Physical machines with different hardware, making disaster recovery on new hardware time consuming.
