the Towerpro MG90metal gear servos seem to be holding up for me.
I use them mainly on throttle on my Nitro Hydors and they take a pounding on the water doing 60-70+mph.
I also have a bunch of the Hitec HS81MG servos I might use instead.

I plan on using a seperate battery for the radio (2S1P 1300mah) pack using a
volt. regulator for 6 volt output.

I did find that the micro servos really don't like any thing OVER 6 volts.