Electric fuel pumps flow more, don't fail as often (in production cars, but hot rods may be different), don't dump fuel into the tank when they fail, and can provide high pressures needed for fuel injection.
Mechanical fuel pumps are simple, easy to replace, simple, don't need a return line to the tank, simple, don't fail if you run the tank dry, simple, and are very easy to replace if something were to happen.

I would stick with mechanical if you have no real reason to convert. They are reliable, effective, and only need one fuel line running from the tank to the pump, and then another from the pump to the carb. None of these fancy pressure relief valves (high pressure mechanical pumps do require one), no complicated plumbing, no extra wiring, just a pump. And one more note, they are very QUIET. Nobody likes that buzz noise many electric fuel pumps make.