I really liked them both but I'll go with Justin because he looked younger and had a better contrast to Dean Cain.

@Stratego! said:

...do you prefer? It's not like Michael Landes was bad perse, but I'm just too used to Justin Whalin. I think Landes may have been the better actor, but he kind of looked too old for the part. Whalin had the right boyish looks.

I agree Jimmy is originally a teenager but as the show went on they had to make him age I just wish they didn't do it so soon much like ending the show too soon.