I have a 4 stage unit from Turbine Products, A401. Very happy with it. I did some comparison shopping by borrowing time on a high end Apollo and a 3-stage (I think it was 3) Fuji.
Other than price, the biggest difference to me was that the A401 comes with a bleeder gun. According to tech support from Turbine Products, I could put on a non-bleeder gun as the turbine has a bypass valve but it will shorten the life of the turbine. After a little practice, I just haven't been bothered by the continuous air flow. In fact, it came in handy at least once as a quick way to blow off some crud that has settled. I spray outdoors so there are risks of settling crud, but so far no bird poop.
Keep in mind that there are extra costs involved with any system. The cost of the needle/nozzle sets for example. In the case of the A401, sets are $30 each. Compare that to the price for Earlex, Apollo and Fuji sets which can push $70 each.
All in, I got a 4-stage rig plus 3 extra N/N sets plus some other odds and ends and shipping for right around $650. For maybe $550 you could load up with a 3-stage unit, extra N/N sets, replacement gaskets, etc.