RE: Kress RK-20 or RK-720???
Hi!
Your OS .25 vfdf would be appropiate in either Kress RK-20B or the RK-720. The major difference between these Kress units is the Fan diameter : RK-20B is 4" and the RK-720 is 3.5". Other differences is housing material and mounting in the housing/tube.
The potential for more thrust is in the RK-720 (more power to turn a smaller fan at a higher speed).