Compared to figuring out power systems, glow IS simple
Glow is simple because it's based on a fairly small set of factors, namely air, fuel, and displacement. There is only so much power to be had from a certain displacement, and engine manufacturers have settled on a de-facto standard for engine sizes. That means one guy's .40 engine is going to perform similarly to another guy's .40, and the engines are going to be of similar size and weight.
Electric, on the other hand, is wide open. There are so many different factors I can't list them in a reasonable amount of space... Physical size means nothing. The numbering systems used by motor manufacturers are little more than fancy model names; they tell you nothing about what the motor can do. Motors are capable of wide ranges of power; the same motor might work equally well at 100 Watts or 1000 Watts.
The best thing to do is start out reading the electric FAQ Dr. Kiwi recommended. When you set up your first plane, simply copy what someone else has already done on that plane or a plane of similar size and weight, with similar flying characteristics. You can kind of "reverse engineer" the system to see what goes into a successful power system, and perhaps even see why those choices were made.