If a person was looking between these two engines with the intentions of mounting it on a Top Flite warbird, you might want the .75. The .75 will run with the Top Flite in-cowl muffler , but it is too restrictive for the .90, and will cause it to overheat. Other than that, the only reason to buy the .75 over the .90 would be if you got a really great deal on it.

Pilgrim