Between Vonleh and Exum, Exum has the greater chance of busting. Vonleh size and rebounding would allow him to have a rotation spot immediately.
Exum also has the biggest chance of being a star. He has a huge risk/reward factor. It's going to take a lot of stones to pull the trigger on Exum.
KOC wouldn't even sniff Exum, DL just might take that chance.
Just like KOC didn't sniff Kanter, who had a similarly sparse track record of playing experience.