Prot warrs since they are much more easier and have had a lot of thought put into them and are easier for agro controll. Blood dks are okay and fun but they don't have as good survivability as warriors and require more skill than warriors.. But it is always good to have a good DK tank. But really it is a matter of opinion
I would recommend a prot warrior just because they have had more thought put into the class because they were the first real tank. They have better abilities for survivability and are easier to play. Blood dk's are fun and can be good but typically warriors tank end raids, or pallys/druids. Not death knights...not enough survivability.