Thanks for the quick replies.
My project involves thruster around 1 meter in diameter. And, generally, at that size you need to use mechanical gear to lower the propeller rpm to an efficient speed. With RDT the numbers of poles in the electric motor can be much higher, and therefore lower rpm are possible at a given frequency.
My interest in smaller rov thrusters is for a small scale prototype of an object for thrusters to pull.
And you are right, the price is the big issue. From what I, until now, have collected, 100mm RDT price is 25 times that of T200. Bringing similarities to BlueRobotics founding history, from the quest for cheaper parts.