I am building a Tiger 60 (Deluxe).  The Deluxe kit is not made anymore but Goldberg will still send the plans and die cut wood necessary to complete the mod for a small price.  I have looked at many retract gear trying to decide which will be the most reliable and also not too expensive.  Mechanical are the cheapest, but I decided to go with air up/ spring down.  Or is it spring up/air down.  Either way, they are made by Robart and you can get them from Tower for about 120 bucks for 2 mains.  Only 1 servo required and you can mount it in the fuselage.  The gear is activated by an air valve which is activated by the servo. 
<font color="#00265e">http://robart.com/retracts/6-12lb510.aspx</font>
That is the link to Tower.  Check it out.  Just another option to look at if you havent allready picked up your gear.