They are both high quality products. The Legend is basically the same regulator as an Apeks, with a few minor mechanical changes. It is different from the Mares Abyss in that it uses a balanced second stage. It also features two different user adjustments on the second stage body.
The Mares Abyss also uses a balanced first stage, but the second stage is an unbalanced design. It doesn't have any user adjustments, so you simply put it in your mouth and dive. The design of the Abyss second stage is considerably mechanically simpler in design. It offers great breathing effort and is very reliable due to its simplicity.
Both regs easily meet the US Navy Class "A" requirements, which means that they can meet the needs of two hard working divers at a depth down to 165fsw.
While I do own Mares and Apeks regulators, I currently have as my favorite reg the Mares V42 Proton Metal. It offers great breathing effort, along with metal bodied second stages. Metal second stages are much more expensive to produce, but offer the benefits of heat transfer in cold water, and the reduction of "dry mouth". The Legend does incorporate a metal air barrel for these same reasons, and the Abyss does use a metal second stage.
Greg Barlow
Former Science Editor for Rodale's Scuba Diving Magazine