Both are good systems from good companies who support their products well, and most people that have either like them- that's why they bought them. The best way for you to tell which you would prefer is to try working with them for a while- interact with them as you would in diving, set up diving parameters, look at the logs, etc. Take them into a dark room to check the lighting. Do this on each. What are they like for you to use? Your personal preferences may not match others. You won't go wrong with either one.
Some much prefer a wrist mount, others (like me) tend to be involved in activities while diving that would make a wrist mount a very bad idea- this just comes down to personal preferences and the kind of diving you do.
The Cobalt has a higher contrast and a higher resolution display, is inherently lit, and it is color, but the much larger difference is in the way the user interface is organized. It is designed to be easily useable without resorting to a manual or a class- in the cliche term, intuitive. This involves a lot of design details in the interface, and doesn't come across when looking at a feature list. This sort of interface design inspires devotion on the part of some users, while others don't particularly care- again, it comes down to a personal preference.
There is a basic difference in how the diving interface is organized- the Luna is somewhat modal. If you look at the compass, that takes over most of the screen for a time- there are different "views" depending on what you select to see in the dive. The Cobalt dive screen stays consistent, when the compass is turned on no information is hidden, and the compass can stay visible for the duration of the dive.
There are some obvious differences, the Luna can track multiple tanks, the Cobalt tracks pressure in only the one tank connected to the hose (though both will track multiple mixes for deco calculations). Luna connects to a PC through IR, the Cobalt through USB. At night or in dark water the Cobalt is going to be much easier to read. Cobalt is fairly new, in production less than a year, and there were some issues- mostly with the button sensitivity and some batteries- early on. I believe these have been fixed.
