Kiermeier is better at everything than Pillar. Not substantially light years better, but better. Pillar is Kiermeier-lite.
Also Pillar is over 1 year older.
I dunno, 8.3 million isn't bad for a 4 WAR player, but in Pillar's case when it's 100% defense and no bat... I really don't think you need to lock him up long term. Defense doesn't drive arbitration paydays. Go year by year, pay him as little as possible and cut him loose when someone better comes along, which won't be long.