The general performance between a magneto equipped engine with the timing correctly set and having the correct air gap is no different from an electronic ignition. A mag equipped engine will be off a little bit in the low rpm band but the vast majority of modelers are not experienced enough with gas engine operation to notice the difference, and they don't operate in that low of an rpm band.

The big difference is that an electronic ignition is much easier to alter the timing for specific uses. Meaning retarded for running on hot engines, or advanced for running fuels other than gasoline, or setting a curve that enables best performance on an engine that runs both hot and exotic fuels. Only a select few individuals have the capability of performing those alterations accurately. EI is also much easier to start since the flywheel (propeller) does not have to travel as fast to activate the spark generation mechanism. However, there are a couple of ignition manufacturers that have designed their EI systems to require a fast spin to excite the spark, negating much of the benefit in having EI.