Hi,
If you have an OS91 use a Ramtec, its designed for it and top end is the optimum for the engine , the Dynamax was designed for the OS77(someone correct me if I'm wrong) it allows about another 2000rpm on the top end since its blades are less dense which is stretching the engine. The advantage the Dynamax does have is it allows for a smaller inlet, eg F16 as opposed to a Trim Spectre, having used both then the Ramtec is my choice if possible. In the end its far more important to spend time setting it up correctly. Mike