I saw Inception in both IMAX and UltraAVX, so I guess I can give a fair comparison. The theatres were Cross Iron's UltraAVX and Chinook's IMAX.
Screen: I don't know what the aspect ratio of AVX is, but like IMAX, it fills up the entire field of vision and has more vertical space than a normal screen. That being said, it's not as tall as the IMAX screen (nor as large, but not by much). As to which one looks better, I honestly couldn't pick one; the picture was crystal clear on both.
Sound: the sound system is where things get really interesting between the two. I guess everyone knows IMAX is loud and extremely detailed. From my experience, the sound in AVX was not as clear (though still excellent), but what it lacks in clarity it makes up for in sheer power. The theatre literally shook whenever there was an explosion or gun shot or reverberation. It was really, really, really loud, MUCH louder than IMAX. I honestly left the theatre a bit shaken, but I loved it.
Other: the seats in AVX are indeed better than that of the average theatre and Chinook's IMAX, but I wouldn't say they're extravagant or anything exceedingly special. Reserved seating is a fucking relief though; I don't mind lining up, but it's really comfortable to just calmly walk into the auditorium and have the seats pre-chosen. Another thing, UltraAVX is $1 cheaper (15.99 I think it was, versus IMAX's 16.99; both prices are pre-tax).
In my opinion, it's a toss-up between the two. If the movie was shot using IMAX cameras, then the choice is obviously IMAX. Alternatively, I think UltraAVX is using RealD technology, so if the movie is in 3D I'd probably end up choosing AVX (I thought the 3D effect in Avatar was better in RealD than in IMAX3D). For most movies though, I'll honestly just end up choosing whichever location is more convenient.