jneesy

Either engine will fly the plane with no problem and have power to spare. What this boils down to is what you want on the plane to fly it. Any 1.50 will have more than enough power. In fact most of the 1.20's on the market will do the job and have power to spare. I don't really understand what advice your looking for.
You could go with a 4-Stroke, a 2-Stroke or small gas engine on the Thunderbug and any of them will more than fly the plane.