I think the XFL is going after the good ol' boy, anti kneeling crowd that are boycotting the NFL. They'll have some initially high ratings, but will putter out quicker. The XFL will probably sign some college stars that didn't pan out in the NFL. Manziel, Tebow, etc. If they learn from their mistakes, they'll be less gimmicky than last time. They'll probably also pander to the "NFL has gotten too soft" crowd.
AAF will have smaller, but much more steady viewership. They also have the advantage of starting a year earlier. They seem to have a long term goal of forming an official partnership with the NFL. An NFL Europe, without the European aspect.