RE: Why buy a Futaba 10C when you have a 9C?
I've only been in the RC side of the hobby for 5 years and started with a Futaba 6XA Super which I used all through my flight training and novice flying stage. I had a Kadet LT-40 with the single aileron servo so only used 4 of the 6 channels.
Once I opted to move up to a 4 servo wing UltraStick, I got the 9C Super with the synthesized 72MHz TX module and R319DPS synthesized receiver (no more crystals!) since Great Planes had a $50 holiday rebate sale on them about 18 months ago. I fly the UltraStick and a small electric 3D parkflyer. Yes, I noted the mix for the full span ailerons in the 9C had a bit of a lag between the inner/outer wing control surfaces.
My teenage son grew bored with the LT-40 so Santa got him a Mini UltraStick guided by a Spektrum DX7. I was impressed enough with the 2.4GHz technology I was considering getting either a Spektrum or Futaba 2.4GHz module for my 9C. Horizon's recent "Great Giveaway" event made that decision a no-brainer but I wound up getting the JR X9303 2.4GHz system which works with all of the Spektrum 2.4GHz receivers as well.
No retro-fitted 9C, no 10C for me. I'll still use my stash of Futaba servos but I'm starting to buy Hitecs now as the need for addition servos presents itself.
To sum up, my purchase decisions were mostly affected by price and popularity. At our field, there are only 3 or 4 folks left still flying 72MHz equipment. Of those who've made the jump to 2.4GHz, about 94% of them are Spektrum/JR.