Well, their lyrical material isn't similar, but the sound is. If Ensiferum are only called viking metal because of their lyrics then they seriously need a genre reconsideration (what exactly consitutes viking metal, actually... I guess I never really thought about that). Wintersun sounds a lot like Ensiferum, only more bombastic and symphonic. I guess I'll call it symphonic melodic metal; because it really doesn't seem right to call it power metal to me. I know earlier V said that harsh vocals don't consitute death metal. I suppose it could be said that clean vocals don't consitute power metal, but something about that just seems weird. Personally, I think we have to take vocals into consideration.
So yeah, Wintersun is great; but I'm calling them some kind of symphonic/melodic metal (whatever... I have no clue).