Worm gear.
Pluses, a single lead stays where you park it. Power off
Down sides, very high friction loss and gear ratio. things may move way slower than you want. I would stay away from FP's on highly loaded arms, they smell real bad when stalled. Consider moving FP's to drive along with a pair of Cims if you are doing tank drive. Andymark
http://andymark.biz/am-0002.html Makes a gear box to match Fp's to Cims. you only lose about 5 to 7%. Almost used one last year As a shooter motor.
Biff