Hi, the reason why there are good deals on the Serpent 733 is that there is a new Serpent 747 being released any minute now.
As for the Kyosho I do not know anything about it.
But from what people on message boards are posting, the 733 was not as good as the MTX4R or the Xray NT1. Then again they also say that the MTX5 isn't as good as the MTX4R, but then these are messageboard opinions.
Personally if I was buying something brand new it would probably be NT1 2012 which should be out soon, or Serpent 747 which also should be out soon.
NT1 is a bit more traditional and is similar to the MTX4R, which is a proven type of design and runs well, and then the 747 is more of a innovative design in some ways. There's a thread on it here in this forum.