Um, where in the South do you live?
Some of this has been said already:
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Branding. Coke is a strong brand with a powerful emotional connection to a lot of consumers. The Coca-Cola Company has done a very good job of shepherding this image over the years. Even the New Coke fiasco was accidental genius: by taking Coke away, the company increased demand and created a protective impulse toward the product. Of course, they had no idea what they were doing, but still…
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Sweetness. Pepsi tastes better in smaller doses, Coke better in larger doses. Also, Coke makes a bad chaser after Pepsi, while Pepsi isn’t bad after a swig of Coke.
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Branding. Coke, as far as I can tell, is the official sponsor of the Olympics, coldness, refreshment, Santa Claus and America. Pepsi pretty much just has NASCAR.
Well, Achernar, I live in Arkansas but make frequent trips to Texas, Louisiana, Alabama, and Georgia. I’m pretty sure that Dr. Pepper is mostly a regional oddity and can’t measure up to the Coca-Cola and Pepsi giants, however, I am still curious about its rank in the great cola pantheon. Does anyone work for Coca-Cola or Pepsi that can give us some stats?
I have never lived in the south and Dr. Pepper has been available pretty much everywhere all my life. I know plenty of people who drink it, including my wife.
I live in Atlanta. What’s Pepsi? 
I can tell the difference between coke and Pepsi, and I also find Pepsi to be disgusting. Coca-Cola tastes like cola is supposed to taste.
At restaurants, I ask if it’s Coke or Pepsi. If it’s Pepsi, I order a Mountain Dew.
However, if I have to drink Diet, Diet Pepsi is way better than Diet Coke. Strange…
Coke tastes good as a mixer in drinks. Pepsi is what I drink.
I wouldn’t drink staight Coca-Cola if you handed it to me. I’ll pay for Pepsi first.
On a slightly related topic, I’ve never understood why Coke and Pepsi spend so much money advertising their drinks. I don’t think there’s a single person in the world who hasn’t heard of them, and I can’t imagine anyone changing their brand because a dancing polar bear told them to.
Sure, if they have a new flavor or whatnot they need to promote it, but 99% of their commercials boil down to “Coke: It Still Exists.”
Personally, I drink Pepsi because that’s what I drank as a kid, and that’s what I’m used to (Coke seems to have a harsh, battery-acid taste, especially in cans).
I remember a special about soda pop history on cable from about a year ago. It talked about the history of Pepsi and the fact that it used to be considered a “Back room drink”, meaning that while you might serve your company the prestigious Coca-Cola, you’d sneak off to the kitchen and enjoy the more favorable Pepsi and was marketed as such. Given that history that Coca-Cola would be branded as the more elite might give reason why it’s sales are higher regardless of taste preference.
—slight hijack—
A couple months ago at work, the vending switched from coke and pepsi machines living side by side in harmony to all of the coke machines gone and replaced with pepsi machines. Frankly, it was a bit creepy. You’d see now 2 or even 3 Pepsi machines staring back at you. Apparently, the vending is owned by the same people that run the cafeteria, Aramark (insert disgruntled vulgarity here). Aramark said after the fact that Minnesota drinks Pepsi two to one over Coke. I looked but never did find any numbers to corroborate their claim.
I don’t care much. I intentionally alternate between the two to stave off boredom. I do this with most foodstuffs and drinks.
Perhaps many persons are like me, and this answers the OP?
New consumers. While it’s true that few people will change their preference, every person has to form that preference in the first place, at some point. As a kid, I started drinking Pepsi preferentially because I thought that the Coke commercials were too arrogant. Nowadays, I choose Pepsi because I prefer the taste, but that preference is probably influenced by all the times that I drank Pepsi as a child. So directly and indirectly, I chose Pepsi because of the advertising.
I doubt it. Coke suffered a tremendous black eye in the New Joke fiasco. The president of Pepsi even published a book celebrating their victory. As it turned out, when Coke Classic returned, Code got more shelf space in stores, and thus actually came out ahead.
As for me, I’m with audilover - Coke or Mountain Dew. And I don’t do diet.
I don’t give a damn about the way they advertise, who pitches the products, or how the drinks are packaged. I’m a coke drinker, and always have been. Pepsi is TOO SWEET!!! I even thought it was too sweet when I was ten, now that’s saying something! Sickeningly sweet, actually. I can’t stomach more than a teeny bit. If there’s pepsi instead of coke at the restaurant, I’ll drink water or root beer.
Also without seeing the actual advertising material it could all be a con.
Back in the 70s Pepsi came up with the brilliant: “In recent taste tests 2 out of 3 Coke drinkers preferred the taste of Pepsi.” Some people thought this meant that 2 out of 3 Coke drinkers prefer Pepsi but of course it didn’t.
How does this work?
Go out and ask people if they drink Coke. Collect 999 Coke drinkers. Split them into groups of 3. Give each of the 3 a blind Pepsi and Coke taste test. Now if 2 of the 3 sat they prefer Pepsi, and this happens in 2 of the 333 tests you can honestly make the statement above even if every other test ends up 3-0 to Coke.
Pretty clever hey.