I think they provide different functions. Kanter is only a Center, while Theis can play either big position well (enough). Gun to my head, knowing nothing about who we are bringing in, I probably keep Kanter, but I'd like to see a role for both of them moving forward. If we brought in a surefire starting Center (or Time Lord made 'the jump'), I'd honestly rather Theis play that back-up PF role over GWill. He has the size and such a nice all-around game on both ends that he just fits in seamlessly with whoever is around him.