I have both and did some research a few years ago as to which is good for what. The Dynamax was designed around the OS 77. The Ramtec was designed for the OS 91. The Dynamax will run with an OS 91. I have a Dynamax in a JHH Cougar with a KB 80 and it runs/fies great! While I was stationed in Whidbey Island I flew with Jim Crane (Since deceased) who taught me most of what I know about DF. He had a Ziroli Panther which he built around a Dynamax. He said getting off the ground was always iffy. He tried several engine/fan combos. He finally settled on the Ramtec/OS91 combo which provided the best performance for the 22lb aircraft. Takeoffs were very scale, except for the engine sound!
The quality of both fans are comparable. Neither is "better" than the other.
The Ramtec/OS91 setup is a little cheaper. You can even get an area rule tailpipe from Trim/Great Northern that is ready to install.
BTW Great Northern is an outstanding company to do business with!
Hope this helps.
Jeff Stultz
Representing nobody