If you use mechanical retracts, I'd use a separate servo for the nose gear. You'll need at least one 180-degree servo for the mains.

Spring Airs solve some problems, by eliminating mechanical tinkering, as long as you have room for the small air tank. They're also nice because they WILL come down if you run out of air. The instructions are very clear, and they're not difficult to install, since your only connections are air lines, and a regular (even a "mini") servo will work fine to operate the valve. Another bonus is that since you have air lines and not pushrod linkages, they're easy to move from one plane to another.

Depending on the size of your model, you may want to do some weight comparisons, since Spring Airs are a bit heavy for a small model.