I've flown identical draggy airframes with each of these engines. The ST .51 will pull a bigger prop more happily than the OS AX.46. A 12x6 is not out of the question with the ST engine. For this type of airframe, the two engines perform similarly.

In a sleek ariframe, the OS appears to have much more propensity to rev and produce speed and thus would have more peak power.

Where torque turning large props is important, consider the ST. For outright horsepower, consider the OS. Both engines have good handling charcteristics.