ReVirt logs non-deterministic inputs at the hypervisor level. RR will be recording these inputs at the Kernel level. So this makes the inability to replay the OS execution the biggest difference.
Interestingly, hypervisor-level logging of this stuff is both harder (because it has the constraints of kernel-level code) and simpler (because the non-deterministic behaviours are fewer and better-documented at the hardware level than the Linux API level!)
I think it's very likely that, overall, recording a single process is substantially more complex to implement than recording a whole VM. (With significant caveats - recording a whole VM with good performance is going to be hard and making it really useful probably is a whole load of extra code)
