I like Fate’s Milestones, actually. I’m playing a longer campaign, and I find being able to change Aspects to reflect what happened very evocative. Also, you don’t necessarily need XP to get better - maybe you just lean into one kind of abilities more and another less, and then you just can exchange that.
On the other hand. Advancement for mages in Ars Magica is a huge, complex endeavour, but to me, it sings, because there are alway so many possibilities! Am I learning from a book? Am I studying the nature of magic? Am I improving my laboratory? Am I inventing spells, or creating magical wonders? There’s never, never enough time for everything (at least not for me). Yes, this can be daunting, and it involves a lot of number crunching and creativity, but I actually like both.