I played Gravity Rush and got the Platinum. In my opinion it is the best game for vita, it is an absolute essential and I am glad she is going to be in all stars.
While I have not played the actual Starhawk game I was in the beta, The game was very fun and I loved the aspect of being able to call in different things such as vehicles, weapons etc. My only concern is from the gameplay videos it seems Emmet might be a tad bit overpowered. Seeing as he can build a turrent and it can shoot enemies. He can drop down new weapons while his turrents are covering him and with each new weapons his supers change.
I haven't played either, and the first time i heard of them was when they were getting added in (they seem like a waste of DLC since they so unknown, they should have picked someone that is known world-wide and not just in Japan or w/e)