I just made the switch from Madden 20 with mods to Madden 21 a few days ago and can't see myself going back. I played my first few games on default All-Pro sliders and LOVED it. It's plays really really well out the box imo (on All-Pro vs CPU at least). I can't speak for All-Madden or User vs User games.
The CPU QB play is a lot better on 21 IMO.. even compared to the M20 mods. QB's roll out in BOTH directions and make throws on the run. The animations are great and overall the gameplay just feels smoother. I was initially stubborn and also influenced about making the switch due to negative things I read on forums and discord about Madden 21.
If we're talking gameplay and gameplay alone.. Madden 21 wins easy imo. I played with mods on 20 and different sliders for the longest and the game just felt redundant. I think a lot of people here who will advocate for 20 have valid points, but also have an attachment to the thousands of collective hours that have went into modding 20, and I get that. Certain things that are better on 20 modded are sim stats, the morale system, stadium upgrades, and things like that.
I knew I was making the switch to 21 after the first game. And after I played a game against Mahomes and saw how he moved in the pocket, scrambled, and threw on the run.. it was a no brainer.