Steven, this is like asking me "Bill, which sports car would you rather drive, a Maserati, or a Ferrari?" Both are great answers, and no matter which one you go with, you're getting a hell of a set to add to your collection.
I think in this case, I'm going to agree with the majority here, and go with the '56 set as well. First of all, your wife is already interested in the set, which is a huge plus. It's something you can share together. Also, when it comes time to add that Mantle, or one of the second year cards, she'll be a little easier to convince when it comes time to pull out the checkbook. 
I like the '56 set because visually, it's just beautiful (but then again, so is the '53 set). Plus, as has already been mentioned, the player selection is superb. I love the '53 set, but collecting it would be a little less enjoyable because so many of my favorite players from that era wouldn't be in it: no Kaline, Aaron, Matthews, Banks, Clemente, Killebrew or Koufax because they hadn't made it to the Majors yet, and there's no Ted Williams or Stan Musial, either.
I also think the '56 set is going to be more affordable in mid grade + condition.
Whatever you do choose, have fun, and make sure to tell your wife how cool we all think she is. 
Bill
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Building these sets: T206, 1953 Bowman Color, 1975 Topps.
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