I'm pretty new at this, but open head means that it may or may not have a removable lid; tight head is generally a closed system, i.e., you'll have to cut the top off if you want to use the drum for a UDS.
One question that I would want to ask the sales rep is whether the drums come with a rust-resistant coating on the inside. I assume that you would want to burn this out and sand-blast the interior clean.
"What are all these charges?" "Just things that we need for the house, dear."