In-game, ferrothorn is almost useless. In competetive battling he is very good, though.
Galvantula is very fast and strong, so he is very effective in-game. I used him in my white 2 team. I reccomend using this set:
item: expert belt
nature: timid/ modest
Galvantula is amazing in-game. He crushes the 8th gym and proves very useful for the elite four