Servo size?
I have flown several large models (53cc gassers and 1.20 glow) with analog servos with no issues for many years. The Digital servos will hold a tighter center at the expense of higher current draw. You would probably notice the performance improvement of digital servos when flying extreme 3-D aerobatics, or in helicopters. No need to spend the extra money just to get digital servos, although most of the larger servos made now are digital. You will never notice the difference in a J-3 Cub. Since you are going electric, be sure the battery eliminator circuit can pump out at least 10 amps if you go the digital route. A stalled digital servo will easily pull 5+ amps. You may want to consider higher voltage rated servos that can take 6.0 to 8.4 volts without damage, Then you could use a separate 6.6 volt LiFe battery for the Rx and servos, with an Opto ESC for the motor fed from a separate LiPo battery. This could save you some brown-outs and subsequent loss of control. The weight penalty would only be about 4 ounces for the LiFe battery.