Thanks. Looks like the biggest and most important difference is 5mm in height from mounting lugs to top of engine.

The poll was unanimous up to the first 14 votes, and is now close to 3:1 in favor of 55AX.

I'm strongly considering the 55AX - I looked at the manuals for the two engines and the same size prop is recommended for each, and as much as I want the sound of a four stroke, I can't justify spending the extra $100; that $$$ could go toward a plane or new receiver or something. Also, while the 56 four stroke is large for some of the above air frames, it is in the lower range of planes I might be considering in the future, while the 55AX will be plenty of power for most any 40 size plane.

thanks guys.