digi has three lines of PT (from consumer to pro): M-Powered, LE and HD.
forget HD for now, it's > $10k.
the LE and MP lines are, imo, too similar for clear branding purposes. LE s/w has a couple more features than MP.
the bigger difference is in the hardware used to run it. you can use M-Audio h/w (and maybe a few others?) for MP, but LE requires one of a handful of devices (e.g. MBox, 002).
IF you're buying the kit from scratch, you'll probably do better with LE, because the hardware includes the software. MP is priced in addition to the hardware.
overall, the s/w is very similar, but as you go up you get more features enabled. but there's no radical change in how, say, audio is recorded or the mix window is laid out.
i use PTLE, fwiw, and i think the s/w is excellent. the h/w is too pricey for what you get, imo.