Avator-Had a hard time choosing between the two. My experience so far is i found the stihl to be a little lighter and with the engines i've had so far more power. Could be just the ones i have. Both good quality.
I found the mac to be heavy but easy to convert-only done one which is a 28cc and not in the ball park with the Stihl 25.4 cc.
Navy Guns-Check the thread direction on the crank and the CC's, if it's a right hand thread and 22+ cc's it might be a winner.
With a Telemaster a little 21cc Echo or Stihl might be the ticket-Just be sure to beef up the plane to handle a gas engine. The featherlite by weedeater, I believe may be a choice. Have a couple just haven't converted one.