I've been running the Beta, and the new VMWare Fusion is pretty good. I've used Parallels before, and I hated it. Very unpolished, installs virtual network adapters that are hard to remove, and creates lots of junk all over your hard drive.
What I would like is better support for integrated graphics cards (like the X1300). I believe Parallels does this better than VMWare. VMWare should also create a WDDM driver for Vista. Improves stability (user mode) and allows Aero glass (which enables the speedier DWM windowing engine)
DWM is faster than the GDI window drawing. As it is now, whenever anything on the screen changes, the windowing system asks every application to redraw. If the application isn't responding, you get rendering glyphs ('dirty' areas filled with white or ghosts of application). DWM only renders parts of the desktop that have changed, so applications aren't as burdened by rendering. Oh, and DWM is built on DirectX, meaning it should use your GPU power and cause less CPU strain. Oh, and it virtualises GPU VRAM so if your GPU is out of memory, it can use system memory or (worst case) the paging file. That's why dragging and resizing windows on Vista is much smoother than XP, and why you don't see tearing on Vista. You also won't see missing textures in games, where the GPU doesn't have enough VRAM to store them.