The better years are the ones that were better cared for.
Really, Jorian, not trying to be flip about the response, but the few differences between each year of the SC are more or less "significant" depending on who you ask and what their priorities are for the car. The best thing to do is buy a good book on the SCs and do your homework. Two books that immediately spring to mind are Peter Zimmerman's "Used 911 Story" (a new edition just came out), and most recently Adrian Streather's "Porsche 911 SC Super Carrera." From there, narrow your field, then hunt for the car that is perfect for your needs. Then get a PPI from a skilled Porsche wrench with a good rep. Hope this helps!
Edward