can't think of any real changes between your 2001 model and the 1999 model you are eyeing. I know they used a composite recoil buffer, but not sure if it was replaced by the time 2001 came around. Some of the more recent Colt's I've acquired do seem to have a bit rougher trigger pulls. Can't say why, but the standard ar15 trigger is no prize anyway, so i'm very tolerant.
Where in Illinois do you reside? I'm near Joliet in Will County.