Russ, The even thou the ramtec and Dynamax fans are 5 1/4" fans, I believe the ramtec is just a smidgen larger in diameter. The Ramtec fan will give you more top end speed and the Dynamax will give you more low end thrust power.

Dynamax fans are better on heavy aircraft and Ramtec's are best suited for lighter planes (speed). Both fans will fly the DF kits out on the market that require a 5.25" fan, if just depends on your application.