Polygon has the momentum and greater ease of on-boarding, they also offer an entire toolset of different scaling solutions. I think ZKSync has worked for a long time but they have proven too slow, we'll have to see though. Also another challenge with ZK protocols, IIRC, is that they need some sort of "key ceremony" where some initial key needs to be destroyed (because it gives access to reading and manipulating the supposedly ZK protocol). That's always seemed really shady to me.