If your wife is getting more serious about wedding and portraiture photography then the 6D is the better choice.
Sorry to be the bearer of bad news, but serious wedding and 6d/5d2 af system doesn't square. You can use it as a 2nd or backup body, but for anything mission-critical that moves this is not the camera you can rely upon. Think of the couple walking down the church alley, your af tracking fails and you have to say "Well, you know, I saved $1000, could you please repeat it?".
There are plenty of excellent wedding photographers using the 6D as their primary camera. Dustin Abbott, who often posts here is one. A local photographer, well regarded as the best in the town I live, uses a 6D as primary and 5D2 for his second shooter and doesn't have any AF issues with these bodies.
I'll be the first to admit the 6D's AF is not nearly as good as the 5D3 or 1Dx, but it's not bad either. Unless you are shooting lots of sports or fast paced wildlife (BIF), the 6D's AF is probably good enough for most uses including weddings.