There's not much difference between the versions, except that LE is a little more expandable if you want to shell out the extra cash - it can run the DV toolkit ($1000). Both M-Powered and LE are expandable up to 48 tracks/full Beat Detective with the Music Production toolkit ($500).
The main difference in sound I suppose would come with the audio interface. I have no experience with the MBox. The Audiophile is a great, reliable card. It's not a top-end interface like an RME or a Lynx but it's been around awhile, has driver support for just about everything (ASIO, Gigastudio & 64 bit drivers + all the mac drivers), and will be more than adequate for learning Pro Tools.
M-Powered comes with an iLok dongle, LE does not - it uses the MBox as a kind of dongle.