this is a huge can of worms, and i only own an RKL, but i'll mention a few points..
the apex has a built in webserver, so you can monitor stuff and make changes from anywhere, the rke does have net, but the myreef software that will let you do remote changes instead of just monitoring is supposed to be out soon, but they've been saying that for like a year now...
the neptune products use a programming language to set up your controller, where the digital aquatics ones use a menu system that is very easy to use.
when it comes down to it they are pretty similar in price and features, and you can't really go wrong either way, it's just a few nitpicky things that people get all worked up about.