Typically, you should dry your bike off at the end of the day. Sometimes water might get trapped inside the frame -- at least this can happen on the big bike.
Particularly for everyday use, I would not worry much about frame material. For travel, people recommend steel because of its durability and its ability to be repaired. So if you are doing a loaded tour in #$)%#$(%*, the steel frame will be more robust with respect to finishing the tour.
It is something to consider, but I would not let it be a big deciding factor.