Between the two, the Cobalt. Better customer service and support, better designed computer in terms of both software and hardware, and able to handle a greater range of diving including planned decompression diving well past 130'. The Cobalt's only limitations are (1) its console form, (2) its inability to handle trimix (could be fixed by a firmware release but AA has no plans to do so), and (3) its UI design choices to limit configurability on the fly during a dive (good for newer divers, a liability for those who might need the flexibility on a dive where bailing to the surface isn't an option).
But if you don't like consoles, then there are better options than either the Cobalt or the Mares. My wife still dives my Cobalt, while I've moved on to a Shearwater Petrel because I prefer a wrist mount and use an SPG rather than the unncessary transmitter/AI computer combo. The easy of use and simplicity of the Petrel are almost as good as the Cobalt and are going to be a lot better than the Mares. If you want to save some $$, you can scoop up a basic Shearwater Predator running OC only mode in great condition for $700 or less...though I'd say get the $950 Petrel and be happier with its size and battery options.
