I think most people would think of Justin as Jimmy. While Michael was first, he was only there 1 season. Justin was there the last 3 seasons, so that's the one that sticks.
I really liked Michael Landes as Jimmy -- I thought it was interesting that TPTB changed things around so that he was more of a peer of L&C, rather than a young gopher type. But in answer to the poll question itself, I think since Justin had the role for 3 years versus Michael's one, I bet most of us (maybe all of us) would think of Justin's Jimmy first.
I didn't really start watching the show until 2nd season, so I missed Michael Landes the first time through. When I finally saw 1st season episodes in reruns it took me a second to figure out who the heck he was. So I obviously have to go with Justin Whalin.