Quirk is 1cm taller than Schatz. I imagine Schatz is more prominently used as he is more experienced as a line out jumper at this level as he has been around for longer.
Schatz' involvements individually are rarely tremendously effective. This isn't based on this year, this is based on watching him since his debut.
Quirk has shown the ability to have strong tackle breaking and carrying runs, and good hits in defence. In addition he has shown in 2014 that he is able to carry the same work rate as Schatz. His fault has been the inability to do both at the same time.
Further to this, looking at Quirk, he has a frame that still has a higher potential for physical development, which will only improve this further.
Schatz best season to date was 2013, but he still didn't show he was really anything more than a solid, hard working back rower. I'm not saying those players aren't necessary, but we have an abundance of them at QLD and it's impact we need out of one.
I think Wilson has a fair point that keeping Schatz offers more flexibility in recruitment, and regardless Schatz was the one already signed and Quirk was the one off contract.