Having played both for quite a while, I think both have their place. Offline, Madden is the champ. Running with the football feels more realistic in Madden and that's a big percentage of the gameplay in any football game. The other big percentage of time spent playing football games is on defense, and Madden takes this hands down. It also wins in special teams, and I could care less about extra features like a crib a month after I have the game. Football games are played for months and months and ultimately, you end up playing it almost exclusively head-to-head against buddies. This is why the gameplay is so important in sports games and the bells and whistles don't mean as much. It's also why NFL players choose Madden. Also, In Madden, if you're good enough, you can bust through the charting that goes on behind the scenes.
ESPN can seem entirely stat-griven. Uh-oh, Peyton has a crappy completion percentage. Suddenly, balls begin teleporting through defenders into the hands of Colts receivers. At least they somewhat fixed how receivers would flat-out drop easy passes simply because the numbers said they should. It does have a lot of shiny objects and the core gameplay is still fairly solid, but some players are far too dominant and it has a problem with one of the most simple parts of video game football, marking the ball.
Online, I would rather play Madden, but its online functionality on Xbox Live is abysmal. I haven't played the PlayStation 2 version yet, but I hope to god that it allows you to go for it on fourth down whenever you please. This is infuriating in Madden on Xbox Live. When you have the fall fourth and inches on the opposing team's 40-yard line, you should be able to go for it. But then, with ESPN, you're still stuck with the funky gameplay but the functionality is tops. Honestly, I haven't really enjoyed playing either online considering just about every game I win the guy unplugs his ethernet cable and 70 percent of the people playing are casuals who don't have headsets. I think I'd just rather play the computer.