The blog that I pulled this from had this titled as "Brand Logo Consistency". That probably isn't the most useful way to think about the comparison. Both are in fact true to the nature of each's brand. Coke's position has always been about authenticity, it is the original, "The Real Thing". Coke's logo should and has always been true to that. It's a timeless and iconic symbol. Pepsi on the other has always played hip, upstart to reframe Coke's position as stodgy heritage. Pepsi is the brand of youth, their latest tag "The Voice of a New Generation" has been the fundamental theme for years. No matter what year it is, Pepsi is that generation's cola. Now that I think about it, the new logo makes Pepsi look a bit like an old tart in hot pants. Somehow she's still getting away with it.
1992–1993: "Be Young, Have Fun, Drink Pepsi"
1997–1998: "Generation Next" - with the Spice Girls.
1999–2000: "For Those Who Think Young"/"The Joy of Pepsi-Cola" (commercial with Britney Spears/commercial with Mary J. Blige)
Even more amusing than the Pepsi logo changes is the list of slogans from the Wikipedia entry on Pepsi. It's like looking through a high school yearbook. Some truly embarrassing, insipid shit.
Mark Borchardt on the whole Coke/Pepsi thing, in an outtake from American Movie (perhaps the greatest documentary ever made.)