Both are great. But Esad is in a league by himself, imo.
I'm wondering if Butch Guice is only filling in for one issue or if Esad is taking a longer break past issue #6. There doesn't appear to be a deadline problem with Ribic.
This choice has no losers (that is, you'd get beautiful art either way) but it's a lot like asking which of your children is your favorite. It's something you should never say out loud; something you keep close to your chest.
That said, just based on his body of work on Thor, I'd have to pick Coipel -- but it's very, very close. Ribic has been incredible/superb on Thor but he's only done 4 issues so far. Coipel's Thor art, I'd have to say, is more classically "super hero" artwork. Ribic's style, even on Thor, is something you'd not be surprised to see on a Vertigo title like Fables or Sandman.
All of this is to say that any choice between the two is practically insane. It's like choosing between Lebron James or Kevin Durant. You're golden either way; it just depends on what you're going for.
I think we should run this poll again in a year's time when Ribic has had a full year's worth of work on Thor under his belt.
Different styles, both great; I fail to see the point in choosing one over the other.