I use both. Probably want metal gears more than anything else, but I use plastic too. Anywhere from 50 oz to 200 oz but I prefer something in the range of 100 oz. Lot of inexpensive metal gear ~200 oz digitals out there around $35. Tower TS-150 for one, Airtronics 94851 (on sale). Both actually the same servo, except the Tower has a Futaba arm.

I always use 60-80 lb test braided metal fishing leader for the wires (if it is wires). Anything less usually breaks after awhile (at least for me).

Flash has a DS8231: plastic digital @ 113 oz
Viper has a 94851: MG digital @ 192 oz
Velox has a 94162Z: MG analog @ 135 oz
L-39 has a Hitec 5625: MG digital @ 131 oz
the old TopCat has a Hitec HS605BB: plastic @ 94 oz)
etc.

Use what you have, change it if it doesn't work so well.