Brock Lesnar wins both.
Ken Shamrock was a pioneer in early MMA and has beaten some fellow legends of the spot, but Brock was the Heavyweight Champion. Brock is also still winning fights, unlike Ken.
Ken Shamrock was a great wrestler back in the day, but he was never the spectacle that Brock was/is. Brock is an anomaly, a unique wrestler who has stolen shows left right and centre.
I've heard other MMA fighters state just how strong a wrestler Ken was, but his skills were wrecked because of the damage that he took during his WWE career.