I think the trend in the hobby these days is to make things more complicated than they need to be. There is no NEED, notice I said NEED and not WANT, for power distribution systems, or metal geared servos, or digital whizbang thingamajigs or servo programmers for 40% aerobatic planes up to 150cc. Less than 10 years ago it was very common, even for sponsored pilots, to fly nylon geared servos in there 40% Extra's, Cap's Edge's. Before the JR8411 came out...the popular JR servo was the 4721. I think it is still a great servo. If you fly a lot, the metal gears will wear and get sloppy faster than nylon gears. Metal gears cost far more to replace than nylon.

On that note...for your 50cc bird, you need a single servo that puts out more than 100 ounces for the rudder. I am not familiar with the 635 or 645 that you mentioned, but something in the 70 ounce or so range will be more than enough for the elevators. For the ailerons...I don't know if the plane calls for 2 or 4 servos, so I can't comment. I would think that 70 ounces would be enough either way. More importantly you need to pick servos with good speed and that are accurate. Numbers on paper don't tell the whole story.

Flame suit on!

Bryan