I've used sync since 2014 because Sync is amazing. The most recent redesign made it the best looking app I have ever used, and that's still true to this day. Also Sync can be as free, I don't have ads because I like the dev and gave him $20. Now I'll continue to use this app for thousands of hours over the next 10 years.
From my brief testing with infinity, it does seem pretty good. I like that the feed doesn't bounce around like liftoff as images load. But sync just has much more customization. If I wasn't willing to pay, I'd probably be using infinity. But I had years of use with sync and that improved experience is worth the cost for me.