At it's most basic Bearnaise is a Hollandaise with the addition of tarragon and shallots that have been cooked in vinegar until dry. Traditionally this would be made first then the "hollandaise" made with the mixture in it, then the sauce would be strained and fresh tarragon would be added. Nowadays most people make hollandaise and add the tarragon-shallot reduction at the end. As for which is better for lobster, personally I would choose the hollandaise. For me, the bearnaise would be too overpowering for the lobster.