There is a Dragonfly with both shaft driven tail and collective pitch, its the 22a,
http://cgi.ebay.co.uk/ws/eBayISAPI.d...sPageName=WDVW
like I said basically shaft=better control/response vs
second motor=more robust but not good on FP helis at best, plus prone to tail wag due to slow motor speed up/slow down time.
If you are talking about a motor on the tail then I would say the smaller rotor with direct drive, it has a quicker spin-up time and a quicker slow-down time plus the rotors are cheaper and I beleive stronger. The added weight of the bigger motor is nearly offset by the weight of the parts you dont need, two bearings, reduction gear, shaft and bigger rotor.