Both LE and M-Powered software use the Digidesign DAE. Otherwise, the program would have to be re-coded from the ground up.
PT LE hardware uses Digidesign's CoreAudio driver. M-Audio hardware like the 2626 use OS X's CoreAudio driver. PT LE locks the Digidesign hardware while it is running, so don't expect to be able to jump over to iTunes or anything without quitting PT first. The 2626 uses the OS X CoreAudio driver and in my experience it is more stable for it, not to mention you can monitor multiple apps simultaneously.
I had a 002 modified by BLA (preamps, analog stages, and clock) and I replaced it with the 2626. The BLA pres are better than the 2626's which are better than the stock 002's. I have not heard the stock 003 pres but I can't imagine they would be be better than the 2626's. Digidesign LE hardware is a joke. The 003 is reportedly much better than the 002, but it is still more expensive than a 2626 + M-Powered and a lesser value and less expandable. At first I was hesitant about using M-Powered, but now I prefer it for a number of reasons, not the least of which is the hardware.
There used to be an 18 I/O limit in PT LE/MP but with the 2626 and PT 8 I can assign and record 26 tracks simultaneously at 44.1kHz and 18 tracks simultaneously at 96kHz. Using LE you're limited to 18 I/O no matter what you do. It's obvious Digidesign wants to keep their LE hardware in the market so they are putting silly limitations on M-Powered like the CPTK and DV/Timecode which will keep post users using LE by necessity. Keep in mind Digidesign is owned by Avid. Now guess who owns M-Audio...
2626:
$760 w/ M-Powered (includes free iLok)
26 I/O (8 analog [8 preamps], 16 ADAT [SMUX], 2 SPDIF)
24/192kHz, external power supply (less noise)
MP is compatible with lots of M-Audio hardware.
vs.
003:
$900 w/ LE (no free iLok)
18 I/O (8 analog [4 preamps], 8 ADAT, 2 SPDIF)
24/96kHz, internal power supply (more noise)
LE is compatible with 001/002/003 & Mbox only.