Personally, I prefer gravity feed for a few reasons, I got a few brushes(4 at last count). I holder with Water trap for all 4.
When I was younger I used to use syphon/suction feed and side feeds neither are bad but didn't suit me.
I find gravity feed brushes easier and qicker to clean and I do my own repairs on them too. The Cup on Syphon feeds at times tended to obstruct my view.
Two of my gravity feeds got a fixed cup and two got removable cups with lids.
I think in the end it is not a question of which is better but which suits your needs and likes more.