Back when I had my Bugeye Sprite, I installed an ammeter, and I was stunned to see how high the current drain on the battery was at idle, at night, with almost everything on. This was with stock lights and such; nothing special in the electrical system. I installed an alternator, a 55 amp one, and didn't really need the ammeter any more, because it always sat close to zero. I was pleased at how well the modern alternator, with its greater power at idle and built-in regulator, balanced the electrical loads.
Keep in mind, that 20A generator provides 20 amps only at speed, not at idle. So, if you sit in traffic a lot with lights on, an alternator might be a good idea.
One of the problems with the generator is not the generator itself, but that primitive regulator box. You can find solid-state replacements for the regulator, and while I have some concerns about some I've seen, they still might be a good option and might help keep the battery charged better. Obviously, they can't provide power that the generator won't provide, but they can balance the load better and keep the battery charged better, so at those times when you're running on the battery, there is more power available. Also, it can keep the battery charged to a higher voltage, which helps to prevent sulfation and will increase battery life.
I've heard some bad things about that alternator that looks like a generator: reliability problems, mostly. This is anecdotal, of course, so perhaps it shouldn't be taken too seriously. Still, I've heard more than one story, and there probably aren't a huge number of these units in use.
All that said, I've rebuilt the generator for the TR4A I'm restoring, and I'll start out with that. If I find that I need the additional power, I may convert to an alternator. I think I will start with an electronic regulator, though.