Lots of debate on this subject, so let me quickly summarize. Get the Reefkeeper 2 if you want a simple, plug and play controller, that is easy to use, and is more aesthetically pleasing. The neptune controllers on the other hand are more complex, and will provide you with more options. Read about the details on each providers website.
There is a small difference in price (approx $20) when everything is added up together, so they are very close in in price. The rule of thumb; get the reefkeeper 2 for a small setup that needs the basics to be controlled, or the neptune if you are going over 120g because of it's versatility. There are a few other controllers on the market, but these are the most popular ones. I personally went with the RKII because it looks with the rest of my setup, very easy to use, and is able to control all my requirements. Like I said, go onto the websites and see what unit has to offer, and guage it against what you want out of a controller.
Hope that helps.