Dave,
You are sure to open up a can of worms herw with this question, but I can tell you my personal pros and cons of each based upon my experience.

First glow fuel/gas
Pros....heavier sturdier airframe, longer flight times (for the most part), allows those who like to fool around with engines to work on them and experiment with different things (fuel blends/props, etc), fairly wide power range, does not need or require the use of expensive battery chargers (but some of us use these field chargers to allow us to fly all day without the need to return home to charge up)

Cons...can be cantankerous and finicky to get needle valve setting where they should be, difficult to tune identical engines on multiengine airframes, messy fuel and longer clean up times, need for starter or "chicken stick", heavier airframes with higher wing loading

Next electrics
Pros...extremely clean and quiet, airframes usually light with excellent wing loading, no need to carry extra fuel to field or flight box, no starter required, getting better everyday with a lot of focus on newer battery technology

Cons...lipo batteries are a fire hazard, prolonged charging times between flights unless you have multiple batteries, smaller airframes restricted to flight in light wind conditions, batteries are expensive (expect to pay almost as much for motor, ESC and battery as comparable 2 stroke engine)

I know I did not cover everything here and others will chime in with their own opinions, but these are some examples of my own perspective. In my opinion there is no reason why there cannot be a mixture of the two, but you don't see a whole lot of overlap where I live.
Tommy