Fusion has always worked better for me, as I use specialized USB devices (JTAG probes) to debug firmware on embedded boards. I've also noticed printer issues with Parallels, its pretty clear to me that support for USB has always lagged Fusion.
I've never experienced any lag within Windows or Linux, using either the latest Fusion or Parallels 7. I don't like how Parallels wants to integrate Windows into OS X. In the 'old days' Parallels was unstable, that seemed to get fixed with Parallels 6.