I have the Mk25/S600 and it is a great breather (I suspect the Legend is also).
When deciding one to buy consider: water temperature, the appearance of the reg,and also the swivel on the 1st stage.
If you are diving in water colder than 40 degrees then I would look at the environmentally sealed Legend 1st sage. However, that being said I dove my Mk25/S600 in 43 degree water and it performed beautifully.
Because these regs are so similar in performance, I would base part of my purchase on the look of the reg. This is obviously personal preference, but I like the Mk25/S600 better.
The only major difference in the 1st stage would be the swivel turret. Personally I love it and could not dive without it (exaggeration). It allows you to customize (to a certain extent) your hoses, and with no swivel you could be forced to compromise (but I have never dove one, so I don't know for sure).
Lastly, I would take a look at what regs your dive shop sells/ is able to service. If they sell one of the regs I would go with that one. Buying from your LDS creates a good relationship with the owner/ employees. Also, driving to your dive shop and dropping off your reg to get serviced is a lot easier than sending it to a dive shop that can.
Both are great regs and your will not be disappointed if you buy either. This is just the tip of the iceberg and soon you'll have to dedicate a portion of your house to diving. Welcome to the fabulous sport/hobby which is diving!