I have owned an Aquatronica (fantastic, but expensive and hard to get support for since they're in Italy), Reef Keeper Elite, and an Apex.
When the RKE first came out, it was riddled with bugs. The first units had capacitive buttons (non-mechanical) that didn't work well, they only had one, third-party developer fixing software issues which meant it was slow to get fixes, and to update the firmware, you have to take the whole system down and upgrade every piece, one piece at a time (this is still true). They got off on a bad foot, and Neptune came in and introduced the Apex during all of this. The Apex was night and day better. Nowhere near as many bugs, came with an ethernet port built in (separate module on the RKE), built in dimming ports. It was a better deal (until DA dropped the price of the RKE accordingly) and most people jumped ship, myself included. My understanding is that DA has since caught up with the RKE and is also working on a new base module called the Archon.
My Apex has been running so well, I'm just not interested in jumping ship again, personally. But I do know there are other RKE users in the club and a recent convert from ReefAngel, too. In fact, his unit is for sale right now... haha!
They'll both control a tank well at this point. I went and stuck with an Apex for the built in dimming and ethernet ports. They also have fantastic support. If you email in, they usually respond within a couple of hours or less. Not always, but usually. And it doesn't matter if your unit is under warranty or not, they'll help you with software issues. Getting hardware repaired, on the other hand, you will have to pay for it if its out of warranty, but that's standard. They also have a really great forum with a lot of helpful people. I ask questions over there and usually receive answers very quickly. I know DA also has a forum, but I'm not sure how their support compares these days. Maybe someone else can chime in on that one.
Brandon