Go with the 55AX. I have both engines. The 70fl will not give you the performance you want and weighs more. Get the Tower Hobbies muffler for $14.99 for the 55AX and use the other $15 for some 12.25x3.75 APC props and enjoy it. The TH muffler will give you about 300 more rpm than the stock one! The 55AX is also much better on fuel consumption and makes more power. The 70fl is good for "sport" type planes, mine is in my H9 40 size Cub on floats and works fine, but it does not have the performance for a profile. Now, on the other hand, if you get the OS 70 Surpass, it would do outstanding in the profile. Don't confuse the two OS 70's though, they are not the same at all!

Tim