In general, I would think that planets would not have a net electric charge at all.
The reason is that planets are constantly being struck by various charged particles (protons & electrons with some metal1 ions). If a planet had a net negative charge, it would repel the electrons and attract the protons & ions; if it had a net positive charge, it would repel the protons & ions and attract electrons.
This process would continue until the charge was balanced. Maybe there could be some minor oscillations between net negative and net positive, but for the most part it ought to be neutral.
1) Astronomers consider any element heavier than Helium to be a metal