ORIGINAL: tkg
A magneto is an self powered electronic system.
What every body is calling an Electronic Ignition (EI) is a battery powered ignition system.
The magneto has fixed timing, the timing BTDC does not change with RPM. There are EI's that also have fixed timing.
For best performance the engine needs variable timing. less advance at lower rpm and maximum advance at full throttle. The maximum advance needed for max hp varies from engine to engine.
An engine does not care how the variable timing happens only about the amount. At idle and full throttle the mech adjustable timing and the automatic timing will be the same.
In the mid range they are a little different, not better or worse just different. Which one is for you depends on what plane your flying and how you are flying it.
90% of the fliers can't tell them apart.
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A few decades ago, you could buy a chip sold by Stihl that could be added to a magneto ignition system and it would provide some spark advance. It has been a long time ago and I forget the exact amount of advance it provided. Folks used to buy them for their Quadra Q35 engines, which were hopelessly underpowered. I don't know if they are still around today or not.
Mechanical advance is smoother, as Ralph has implied, since the timing advances a little bit with each incremental advancement of the throttle. Most electronic advances are choppy. Some only have one advance which occurs at 4,000 rpm.
The downside to a mechanical advance is the danger of getting kick back when trying to handstart it simply by leaving the throttle open. Brison is famous for using mechanical advance. Mine worked flawlessly. Just don't forget to take the throttle back to idle or you will get slapped by the prop.