I'm using PT LE (M-box), and I find the DV toolkit to be almost indispensable. The last time I checked, M-powered did not support the DV Toolkit, which seems reason enough to spend the extra money for full-bore LE.
Between the three you listed, Pro Tools takes it hands-down. I'm used to sitting on a fat TDM rig so couldn't advise on the DV Toolkit issue, but that should be checked out before you buy anything. If you ever get stuck, there's loads of people that are super-fluent in PT, which is more than can be said for Audition and SoundForge. Digidesign only makes Pro Tools, while Adobe and Sony have bigger fish to fry.