Ext4 is faster in most tests I've seen. But ZFS has many more features. It is not just a filesystem. It also manages your disks (things like RAID, snapshots, etc). So more like Ext4 + LVM, plus it also has additional data-integrity features that are rarely found elsewhere.
I use Ext4 + LVM on my home lab setup because I'm familiar with that configuration. But lots of people swear by ZFS and of course Proxmox supports it "out of the box".