Well...
I own an Aquatronica on one system, an Apex in a box (New system), and an original ReefKeeper.
I've had really a great experience with the AQT; stable, fairly intuitive, it just seems to have been left by the wayside by it's original designers. Not anything bad, just not a lot developing new for it. I wanted I could control from my Android phone.
The APEX does that, but at a cost. I think Neptune's programming language is powerful, but bulky. The ability to add aftermarket open/closed switches to generate scenarios (Like float, water sensor, etc) gives you piles of flexibility, but no framework. It has some nice features, like the web-server built in, and a beautiful display. Multiple dsiplay options will be nice, if I ever drop the coin for the second unit near my sump in the basement.
Adding modules (PM2, PM1, Breakout boxes) gets pricey quick. I'm well into the 4-figure range, and need to spend more. It's a great tool, but it's got a steep learning curve, and cost curve. With that in mind, though, Curt and the Neptune forums are top-notch, and there's piles of user advice out there for free.
My ReefKeeper (And my buddy's RKE) are great units. Bulletproof, easy to program, reliable, but limited in their scope versus a Neptune or AQT.
My first question is, what do you want/need it to do? If it's basic stuff, go Reefkeeper. If you need all the stuff in the Apex, do it, just be prepared to work with it for a while (Why mine is still in a box; once I have a free saturday to program it, it'll come online...)
Just my $.02... If I can be of help, feel free to ask...
-Andy