I came into LNC in re-runs of S2, so Justin's take on Jimmy was the first I saw. And I'd already seen Justin guest-star in one of my other favourite shows, where I'd been mighty impressed with his acting skills. So I guess I was inclined to be biased toward him to start with.
When I caught up with Michael though in S1 I didn't miss Justin or spend time comparing them - I enjoyed Michael's portrayal in those episodes, just as much.
But I do think that Justin's Jimmy was more in line with what I expected the character to be. Slightly naive, endlessly and irrepressibly enthusiastic, young and impressionable. Michael's Jimmy was always just a tad too worldly-wise for me and seemed older than the character I'd always thought of.
LabRat