ARAB is pretty boring to me. It's a lot of back and forth that's super unbalanced because progress is based around capping the enemy point more than kills. Even if you win team fights there are situations where you will get nothing out of it because you can't cap in time. Realistically, the team that respawns has a huge advantage, and the nature of the game without much protection and poke being less valuable makes things pretty predictable. The back and forth is all very fake. Things last much longer than they should because teams are always in the game. You are just sort of throwing yourselves at each other over and over again.
Proving Grounds is a much more dynamic experience that lends itself better to more champions, and opens up more alternative builds because you actually have access to all the items you should. It focuses more on team fights and interesting play than throwing yourself at the enemy for 40 minutes and hoping for the best in ARAB.
Dominion items are on Proving Grounds with SR items that were pointlessly removed like Banshee's Veil. It has lots of stuff except for annoying things like GA.