ORIGINAL: Kweasel
The size of your aircraft is a big factor. Its difficult to get any of the 25cc gas engines to match the power and weight of common and cheap to buy glow engines. The performance difference disappears with the lightweight 40cc, and most all of the 50cc and larger engines with electronic ignitions. As far as reliability, it depends a lot on the individual. Its usually easier to get 100% reliability form a chainsaw than from a glow engine, right up till you get rf interference and lose a plane. That may buy years worth of glow fuel.
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Your glow engine's muffler can loosen and do the same thing (glitch your receiver to destruction) to your glow model.
With the latest offerings from China, the 26cc gasoline size is now coming into its own. The old 40cc lower limit is being shoved downward by progress.