I have to say -if you are powering this for ANY kind of serious aerobatic performance, and you want accuracy, go with FOUR. It is too easy to do the way they've designed the wings, and you can "Y" both sides together to make it work with only two channels...
There is a LOT of benefit from having the extra servos.

That being said - your question was "what kind?"...if only using two get some VERY strong, probably coreless, 2 bb servos. The flavor is up to you...Futaba, JR, Hitec, Airtronics, Cirrus, GMS, Multiplex, and who knows what else.

What you need to be *happy* with the Pitts is ailerons that respond immediately (speed), center the same every time (deadband precision and centering), move to the same amount of travel (gear train/potentiometer), and HOLD the control input through the performance range (torque).....

My choice - for CHEAP - GWS BP105's (2bb, metal geared, coreless, 110 oz) Not the fastest, and centering isn't as good ast my BEST choice - JR 4721 - 2 bb, nylon gears, coreless, fast, about 110 oz. If you have LOTS of money - four DIGITALS, pick your flavor.