I would say a major feature for me that sync has is to be able to hide viewed/interacted posts on button press.
I also have bugs with connect not marking viewed/interacted posts as read.
But I keep both in hopes that Connect get polished overtime, i mean it takes time to get everything right.
I was not a Sync user on Reddit but LOVED Relay for reddit. Anything that navigates/looks close to Relay I will gladly pay for.