I use pens and it rains on us quite a bit around here. Heavy rain problem is never the ink but the wet soggy paper. I carry two pens ALWAYS (1 in pocket and 1 in my sock). I generally have at least 3 spare pens in my organizer on the sideline. I am in agreement with all around here
1. use what works best for you
2. ALWAYS HAVE SPARES
That being said, I have had to loan the following to other referees who weren't prepared: pen, score sheet, shirt, cards and a whistle (still can't believe that one. Wasn't because his lost function. He just didn't bring one). Just be prepared and have backups for for worst case.