The Guardians of Order game produced in 2005 used both the D20 rules as used in D&D 3rd Edition (but heavily modified and improved) and the Tri-Stat rules. Only one rulebook was produced, but it was very, very impressive, easily the finest RPG rule book I've ever seen, with some fantastic artwork. Only the recent PATHFINDER rulebook came close to matching it. The rules for both versions were very good, it has a fantastic map and there's some great 'bonus' worldbuilding material contributed by GRRM that isn't in the novels yet.

The newer version produced by Green Ronin in 2009 uses a d6 skill-based system instead. The rulebook is vastly inferior in quality (with some pretty bad art). The rules seem to be adequate, and it has a big bonus in that it has a BIRTHRIGHT (old D&D 2nd Edition campaign setting)-esque element to it where the characters create a noble house from scratch and guide it through the politics of Westeros at the same time they are having adventures. However, this is easily exportable to the GoO edition as well.

IMO, the older GoO version is the better one, but the Green Ronin one has the benefit of being better-supported (GoO went out of business shortly after the book came out) with more rulebooks and expansions planned.