Definitely Austin Air. My Phoenix had such sinus problems. We bought an Austin Air Allergy machine, and he has not had sinus problems since! We bought it from Parrotsparrotsparrots, she gave us a really good deal. We also cleaned our air ducts, which helped a lot too.
They are not cheap, but they have a 5 year warranty and filters have a 5 year prorated warranty as well. We had cheap air purifiers, with HEPA filters, and they didn't do anything. We replaced filters once a month or more, and at $25, we spent more than we would on a Aunstin Air filter, since they last at least 4 years. I can actually smell that the air is cleaner. It is hard to explain, but it works wonders. I had a 2 month long sinus infection that cleared up 2 days after we got the Austin Air.