There's a first-look preview up now at the Design Mechanism right here. It looks really well done, and I am getting very eager to land a copy of this. I appreciate all Mongoose is trying to do with Legend and all (and Legend is by definition a fine system), but RQ6 is shaping up to be the most artistically well-done and comprehensive edition of the system so far.
In any case, even if one decided to opt for RQ6 as your primary edition of the system, I think that Legend's compatibility will work in its favor. I am sure I will be able to use Blood Magic (my new favorite supplement, hand down) with RQ6 as easily as Legend.
And then Loz mentions a reworking of spirit magic and the addition of a fifth magic system, mysticism, over here. Oooooh! Too much excitement.