I'm going to have to go with ML here
While I don't think I'll ever get tired of the original 13, them being able to interact and tell new stories with a bigger cast that I've also grown to love has been a huge breath of fresh air. And while I don't care for the actual ML gameplay, the characters and music is where it sells me.
The (relatively) low number of characters allows each girl to be developed much more than your average member of the Cinderella Hordes, and it pays off. Each character has at least a little more to them than their initial gimmick, which is more than some can say. The unit dynamics are especially amazing imo, and I don't think there's anyone I really dislike - only characters I don't pay that much attention to like Tsubasa, Rio and Tomoka. My only big issues with ML are the often horrible fanservice and that the main trio hardly does anything new compared to... any other main trio in the franchise.
The music itself is also incredible - while there are a lot of songs I'm not a fan of, the ones I do enjoy are some of my favorites ever. There's some real beautiful songs here like Praline, Flooding, Ehon and more, while songs like Emergence Vibe, Up10sion and Super Lover are some of the most outrageously fun songs that the series has produced - and there's such a variety besides that, each compimenting its singer/s really well. I've had a great time delving into the ML cds, and it gains a lot of points for always trying new things in every album - the duets especially. Again, there are some lackluster and outright bad songs in ML, but it's overall consistently great.
CG feels like it's Whole Own Thing and while I like a lot of the characters, I've never been able to really love the branch itself. I wish I was more into SideM, and it's just a matter of time until I am, haha. ML, however, feels more like an addition to the original premise, much in the vein of DS, and idk - it just has that Something in it that I love.