I have owned both and here is post I made a couple months ago:
The RK2 has the following pluses in my book: <ul>
<li>It is metal powdercoated rather than a plastic box. It has a sturdier feel.</li>
<li>It is much easier to program the RK2 from the head unit and ultimately easier to use</li>
<li>Even compared to the new fresh looking AC units it is prettier to me</li>
<li>The head unit is a bit smaller and easier to mount</li>
<li>The head unit only has one cable coming from it also making it easier to mount</li>
<li>The head unit has mounting brackets built in so you can screw it to whatever you like as opposed to using velcro or creating a container to mount the head unit</li>
</ul>The AC (and this is in general for the ACJr and AC3): <ul>
<li>Programming rather than static channels. This allows a single channel to work with multiple conditions. A channel can be shut off via temperature, pH, a timer and can oscilate all at the same time. As opposed to a channel only having one function. Also, you can set feed mode on any channel you like rather than on a select few. The unit is just much more flexible which of course adds to its difficulty</li>
<li>The ACJr can control 12 devices versus 8 on the RK2 and the AC3 can control 24</li>
<li>The processing is in the head unit seperate from the power distribution block so there is no interference with power supply into the temp, ph and such. If you don't have a noisy ballast this won't be a problem, but you likely won't know that until you have a problem.</li>
<li>Rebooting the head unit doesn't effect the power supply to your system. On both units, I have run into occasions where a reset of the head unit corrected a problem. On the RK2, that means everything shuts off and back on. On the AC system, only the head unit reboots.</li>
<li>Digital input so you can monitor float switches, water sensors, etc.</li>
<li>Each interface is seperated so you don't need a special splitter and you don't have the headaches associated with such</li>
<li>When plugged into a PC, noise from regulated DC power supplies i.e. laptops won't cause problems with the temperature as they seem to be on seperate insulated circuits</li>
<li>True data logging support. If you don't have the controller plugged in at all times, it will still log data for quite a while in most cases well over two weeks.</li>
<li>Ability to vary temp and day lighting seasonally</li>
<li>Moon cycle simulation</li>
<li>Ability to produce random effects such as wave making and even random lighting changes</li>
<li>pH probe temperature compensation</li>
<li>Password protection</li>
<li>True telnet interface so you can view AC data via any standard telnet program including the one that ships with Windows and you can turn ports on/off via this interface</li>
<li>AquaNotes expands the functionality by adding webserver, email alarms and the ability to configure your unit from a PC.</li>
<li>You can save off your programming between firmware updates. Huge pain to setup every channel every time a new firm ware comes out.</li>
<li>You can give channels 3 character names versus having to know what Channel1 is</li>
<li>Larger more stable company so support isn't a big issue. I waited several weeks to get a new unit when mine was having trouble as DA had none in stock</li>
<li>Curt (the owner) often picks up the phone when you call Neptune. I like Scott at DA, but I don't know who the president is over there. Curt knows his units forwards and backwards as he helped design/program them and ultimately is responsible for the company. I have dealt with Curt on some highly technical material and I can tell you he is very sharp.</li>
<li>Expandablity. The memory in the Neptune systems is much greater than the DA units so new firmware patches can do more in the future. This is particularly helpful in the programming department as the AC can get new programming commands added via firmware and that usually takes more memory</li>
<li>Expansion modules. Right now it is limited to the LunarSim, but in the works is a Tunze Stream controller so you can control wave functions timed with lighting and such right from your AC. The AC systems use a daisy chaining system so the head units plugs to A which plugs to B which plugs to C so you can daisy chain a variety of equipment together. DA doesn't have anything like this... yet.</li>
</ul>Some people tout cost. If you are looking for a bare bones setup, you can get a cheaper AC unit. If you want to do a PC interface, they are pretty close in cost. About $290 buys you a black backlit ACJr, $270 for the older blue style. Around $330 for a system that has a serial interface with cable. Same system from DA will run $320 and another $10 for the splitter without a long cable which is $5-$10 at Lowes. Configuring the PC interface for the RK2 can be a bit tricky and the PC software for the RK2 does not allow you to adjust programming and likely never will. So if looks aren't a big deal to you and you don't need a serial interface, you can save about $50. $30 if you want something a bit sleeker. Pretty comparable once you move into the PC interface, but again as of now AC has far more functional software.
I really like my AC3 which is far superior to the current RK2, but it does cost more. From my experience with the RK2, had I to buy a new unit today I would buy an ACJr or AC3 as I prefer the flexibility but I cannot fault anyone wanting a nicer looking head unit that is easy to program. I will say if you plan on connecting this to your PC, it is hard to pass on the AC line of controllers.
