Aerc has filters and builtin terminal support. This allows cool syntax highlighting for displayed emails and also support for previewing unusual formats. Aerc has well-made multiple accounts support and tabs out of the box, while with Mutt I had to keep two separate muttrc files for my two accounts and create bash alias for them. Aerc can filter the emails in the current folder and show only the ones that match an expression, I could never do that it Mutt. And overall, it just feels more modern and reasonable, even its config file format feels better.
I had a few things I found weird in mutt and never could fix them through config. For example, PgDown in Mutt goes to next email when I reach the bottom of the current one. Or deleting email keeps it displayed in the list (making you spam $ after each deletion) but doesn't allow navigating to it to undelete (Mutt devs want me to select emails to undelete using regexp? seriously?). I kept having wtf moments like this with Mutt all the time, I almost never have them with Aerc.
Some things still need polishing in Aerc, but it's improving fast.