I think it depends what you're looking at.
Thrun has the ability to body guys on the boards and create loose pucks, and when he's paying attention, he has the ability to clear out the net front. He's slow as hell (hence my Douglas Murray comparison - he really has the same skating stride and tempo), and he has just passable puck movement skills in the OZ but isn't great, just OK on the breakout.
In contrast, Thompson flashes more because he makes great passes, is decisive and effective in the OZ, but the things that don't pop (away-from puck boxing out at net front, board puck battles), he's getting buried out there. They're not really replacements for each other anyway (LD/RD) and play different roles. Overall I think Thompson has a higher ceiling as a mid-roster guy who can have impact moments, but Thrun may have a clearer path to a bottom six "zug zug" Rutta-type defenseman, which you really do always need 1-2 of on a good team.
EDIT: Jeez, all that on a Graf thread, sorry. I hope Graf gets his chance, but I agree he should keep developing in the AHL. maybe he, Cardwell, and Bystedt can make an effective kids' third line like the old Ricci-Thornton line which could check but could also chip in their fair share of goals.