I have seen them both
I saw Owning Mahowney locally in Santa Barbara.
I saw Stuey at Cini-Vegas at the Palms last week.
Stuey was fabulous -- as good a film as I have ever seen in the genre. The boy actor who played young Ungar was the weak point, but everyone else was quite strong. The director was at the showing and gave a nice little intro, and there was a Q.A. session afterwards. I thanked him personally for not having any explicit violence in the movie (there is a lot that is implicit). Also, the Cinematographer was there ... a great job, huge contrasts -- for example between the wide open countryside of the desert outside L.V. and the small hotel room Ungar died in.
Owning Mahowney was a must see, but does not ultimately satisfy.
--Eliot