In my experience, glow is better for smaller engines, say 1.40 and under. (That is a very general rule in my opinion but not set in stone) Without all the extra ignition parts, glow engines are lighter and make more power per a given displacment running on methanol/nitro-methane than gasoline. So I usually would never consider trying to use a gasoline engine on a .46 or .60 size airplane. I've seen people bolt gas engines on some planes and all the extra engine weight and work to acheive a good CG made for a poor flying pig of an airplane.

As engines and planes get larger though, gasoline begins to have a clear advantage. The engines can be priced the same or even less than larger glow engines. The wieght usually isn't a factor with the larger engines and airframes. The gas engines make great power for their size. Gasoline is less expensive and more convinient than glow fuel. Gasoline engines are also generally very consistent and reliable. I would almost never consider a 1.60 or larger glow unless it was just the absolute best option for the application.

Like I said, that is all a general rule of thumb. There are always exceptions. You'll run into some smaller airplanes that have big, round cowls and sometimes require extra nose weight to acheive proper CG. Some biplanes are an example of this. In those cases, that gasoline engine might be just the thing. There are also smaller and lighter gasoline engines coming onto the market. Magnum is even releasing a gasoline version of their .52 2-stroke glow engine. It sounds interesting but there will be the added weight of the ignition and extra battery pack. I'm sure it wil lbe a good fit for some airplanes though.