If they're in the same group then it'll be which ever is at the top of the periodic table. If the two options aren't in the same group then it'll be which ever has 1 or 7 electrons in it's outer shell, if neither have then it might be that one has 2 or 6 electorns in it's outer shell and the other has closer to four.
Sorry I'm really tired and my brains scrambled because clocks in my house are saying three different times, else I would tell you the actual answers.