I think if you get past the on field aspect (in which APF completely trounces 2k5), 2k5 has a much better single season campaign. First of all the weekly prep is a very good feature. It wasn't perfect, but it added a very interesting element to gameplanning. I'm surprised it wasn't implemented into APF. Imagine if you could have used the weekly prep to possibly give Legends an updated ability, like having Christian Okoye spend extra time working out his arms for several weeks so he can get the Arm of Steel ability. It would add a balancing act to the game. If you work out your players hard you might be able to get new abilities for them which will increase your success during the season, but you also run the risk of them getting injured in game or having them fatigued for a game.
Secondly, 2k5 has a better single season campaign because of the weekly wrap up show. This feature brought presentation to a whole different level...it was so cool to see highlights around the league and be caught up to speed on all the week's past games.
APF definitely has the best gameplay in any football game to date, but I'll agree with those that say 2k5 sets the bar for football gaming in many ways. Once the NFL season starts or if APF grows a little stale, I might play some 2k5 again.