I find that AP plays how I think it should. AM might be more challenging, but I've always felt that ratings just matter more on AP. Now, you also need to find a good balance with your sliders, first and foremost for challenging gameplay. You want a challenge, but you don't want cheese either.
With the sliders I'm using (Armor's sliders with some tweaks/normal speed), if I play as the Saints I don't have much of a problem controlling a game against most teams, but I wouldn't say it's too easy.. especially against the really good defenses in the game.
But this year, especially in offline CFM (when they fix sim stats), I'm playing as the Rams. I've been playing with them since I got the game and put Shaun Hill in place of the injured Sam Bradford, and it's definitely hard to get some momentum going on offense against a competent defense. The one thing the Rams do have that is great, is the best D-line in the NFL, good LBs, and an okay secondary (the weakest link, but they have some young up and comers).
On offense you basically have Kenny Britt, speedster Tavon Austin (can be a great weapon), Brian Quick (he is okay), and TE Jared Cook (surprisingly not too bad), but the one thing you need is a better QB to run the Ram's offense, otherwise yes, the game can be difficult playing good teams on AP... but the Ram's defense will keep you in games as well.
My main team as life-long fan is the Saints, but I wanted a slight re-build job for my CFM this year, as the Saints would just be too easy to win with this season. Armor's sliders with my tweaks (and playing on normal speed) has provided satisfactory gameplay for me. Also, I'm not playing with the QB edits, so take that into account too.