It's a decor issue. Either will store your wine well. Just choose what works for where you want to keep it.

When it comes to wine, whatever looks attractive as long as you keep the temp on the cooler side and maintain it.

Metal conducts heat, wood doesn't. And since storing wine in warm temperatures is bad, I'd usually opt for wood. I'm thinking that if you had a warm room, but it cooled off at night, the metal wine rack would keep the wine warmer longer. Just a thought.

wood because you have no risk of it 'heating up' or conducting heat AND it looks nicer.

I think a metal wine rack would be really tasteful. They're so...gothic. They remind me of Edgar Allen Poe, Im not sure why. Probably something to do with The Cask of Amontillado (Hope you've read that or you wont know what Im talking about). With reference to heat conduction...come off it. The amount of heat conducted would have to be extremely negligible, or why would they even make wine racks out of metal? You're obviously not planning on placing it in front of a furnace or anything. Metal, definitely.