Sampson
At the end of the day, Kelvin's consistency wins out. They won nearly an identical percentage of games. Kelvin was about 4% better in conference games, in a tougher conference, IMO.
While Tubbs might have had more "high profile" teams, he still only took 1 to the Final Four, which matches Kelvin. In three of Billy's last 4 seasons, OU finished outside the top 3 in the conference. Kelvin only had two teams not finish in the top 3 of the Big 12, again, a tougher conference.
Kelvin coached 12 years, and never missed the postseason play, and only had one NIT season. That is remarkable. Tubbs coached 14 seasons, and while I don't have the numbers, he certainly missed the Dance and the NIT more often then Kelvin did.
I think it's a close call, probably coming down to what is most important to you. That said, the one thing I always loved about Kelvin was his consistency. He had OU at the top of the Big 12, and in the Dance, nearly 100% of the time. At a school like OU, I think that is important. We aren't known for running top 10 teams out there. Our program was ranked in the top 15 for the past 30 years (or whatever) based on our consistency. And that is largely due to Kelvin.