I have both but have used them on a RANS S7. My engine is a 105hp Zippered ( originally an 80 HP) engine. I think either one would serve well. Only reason I changed was due to wanting to have the 'latest and greatest'. Plus, the 70" was developing severe cracks in the outer finish that I wasn't sure was safe. Whirlwind said it was only cosmetic... I say BS!

The 75" pulls a little harder but I never really put enough time on the 70" to give you an honest comparison.