I don't like high loft synthetic insulation while active (down is even worse). It doesn't breathe well, the face fabric feels clammy, it holds a lot of humidity inside and the insulation suffers under pack weight. Fleece is rugged, breathes well, feels comfortable even when damp. It's so much better while active and particularly so in the winter when moisture build up from inside is your enemy and you need a clothing system that promotes wicking and transfer and doesn't hold humidity. Fleece is heavier and bulkier but that doesn't matter while you're wearing it. It does matter while it's in your pack though.
Fleece + windshirt is a very versatile combo. Apart from the obvious use in winter, I'm now back to fleece in 3 season, a thin pullover at just 187 gr. (similar to a full-featured base layer), it's not even heavy. It's just a bit bulky in the pack but it teams up perfectly with the other layers for a super versatile active wear set.