I would go DI. It's a proven system and it's more common so parts are easier to find. I think the AR got a bad reputation in it's early years, but those issues have been fixed. Just make sure you buy a quality brand. There's a lot of junk out there.
Here's a good article on how a DI AR performs when dirty:
http://www.bravocompanymfg.com/v/vspfiles/assets/images/filthy14_oct10.pdfAnd here's a good chart comparing different brands of ARs:
http://spreadsheets.google.com/pub?key=pwswheghNQsEuEhjFwPrgTA&gid=5