Both actually. I'm focusing on keys and semikeys first. I have a whole bunch of warn common dates passed down from my father, and those fill about half the holes. Once every hole is filled I'm going to upgrade them. I'm also buying a few here and there of the teens, twenties and thirties. Individually those aren't so bad, but in EF+ it definitely costs over time.
My goal is to have every Lincoln ever made. 1909-1958 all brown EF or better (commons and semis). My EF+ rule is exempt for keys, but they do have to be certified. Next I want to complete 1959-Present, all BU Reds. I haven't looked at memorial cents too closely, but the only painful one I can think of is the 1972 DD. The 1955 DD will also have to come much later if I want to be stay married :)