Lots of good comments on this issue, but I feel the need to clarify a 
few issues.  First, there seems to be a consensus that EI provides a 
"hotter and longer" spark.  sometimes true, but the most common EI 
product is a capacitive discharge (CD) system.  The inductive 
(Kettering) system stores energy by using the inductance of the 
ignition coil and the spark occurs when the current in the primary is 
interrupted.  These system, which include magnetos, are characterized 
by having a relatively slow voltage rise time, but a fairly long 
spark duration at a low current level.  A CD system stores energy at 
high voltage in a capacitor and the spark occurs when this is 
discharged into the coil.  Since the coil isn't required to store the 
total spark energy it can be smaller and of lower inductance.  The 
result is a very fast rise time, high spark current and a very short 
duration.  A short-duration spark works fine as long as there is a 
combustible mixture in the spark gap at the moment of the spark.  CD 
systems work very well in engines that tend to foul plugs, which is 
why their initial popularity was in 2-stroke engines, but they 
typically don't work very well in engines that operate with "poor" 
mixture preparation, such as automotive engines running a lot of 
EGR.  I see no problem using CD technology in aircraft engines as we 
don't typically operate at very light loads with EGR, except at 
idle.  The CD system is far more electronically complex than an 
inductive system, which only requires an electronic switch to 
activate the coil.  The automotive guys have spent billions 
researching ignition systems with the result that all automotive 
engines worldwide still use the original Kettering inductive system.  
The CD coils weigh less, but the electronics weigh more.  I agree 
with Walter that the higher spark current will result in a faster 
initial burn, creating an effective timing advance compared to a 
conventional system.