This is related to my other thread "need engine recommendation". I have a collection of FP engines, several in each size from 10 to 60, leaving out only the 35. I recently built two identical Small Wonders (15 size parasol wing from RCM plans) for myself and a friend. I put a15 FP on mine and a 400 MaH NiMH battery. My friend put the MDS 18 and a standard NiCad Rx pack in his.
I don't like to spend money, so I don't go nuts on small radio equipment. Both of us used three standard servos for these planes. My plane weighs 36 oz ready to fly, and his weighs 44. I'll estimate an extra ounce or three for his aluminum gear and axles compared to my wire gear. I weighed the batteries, and his weighs 2.5 oz more than mine. The rest will have to be attributed to the engine. Although the size of the head and cylinder look the same, the MDS has a bigger carburetor, bigger muffler, and ball bearings. You may think that it's all the same either way, but that's just about a 20% difference in weight. My plane has the same rate of climb and is noticeably more maneuverable and less oily. His plane flies great, but I prefer mine.
I really liked the MDS, though. It was quiet, very user friendly, and it spun an 8x6 prop nicely. I'm thinking that maybe it would be a fine choice for a 20 size plane, but clearly it was out of my range for the15 size plane.
I just thought that this was an interesting comparison for anybody who wants to consider different aspects of engine performance.