I would go with either the Omni or the Iwata. The Iwata HP-SB is a side feed system vs. the permamently mounted cup on the Omni. Side feeds allow you to spray in a vertical manner, as long as you rotate the cup to match. Top mounted cups restrict movement and can interfere with sight lines. The HP-SB allows left or right handed mounting of the cup for the south paws.
The Iwata represents the best value, as long as that "used once" comment doesn't mean "farked it up because I dropped it". The current price on Dixie Art is $180.00 delivered in the lower 48. Unless you actually meant the HP-BS which goes for $109.97. That's a smaller top-mounted cup AB, and is still a good deal. A $130 drop in price for "used once" HP-SB seems a bit too good to be true!
The Omni 4000 has a larger color cup, and is listed at Dixie Art for $79.00 delivered, but is also shown as currently unavailable.
Neither would ship with hoses, unless the used one does.
I've heard of people being happy with the Aztec, and I've heard as many (maybe more?) people unhappy. Everybody likes Omnis and Iwatas!