I made this guide a while back:
2001-2005: ABS or Non-ABS
Early 2006:
Level 1 (P): Premium Audio
Level 2 (HP): Add Comfort Package
Level 3 (HPN): Add Navigation
Level 4 (HPNA): Add ABS
Late 2006-2008:
Level 1 (P): Premium Audio
Level 2 (HPN): Add Comfort Package, Navigation
Level 3 (HPNA): Add ABS
Level 4 (B): Add Airbag
2009-2010:
Level 1 (HPM): Premium Audio, Comfort Package, TPMS
Level 2 (HPNM): Add Navigation/Weather/XM
Level 3 (HPNAM): Add ABS
Level 4 (BM): Add Airbag
2012-2017:
Level 1: Premium Audio, Comfort Package, TPMS
Level 2: Add Navigation/Weather/XM/Trip Planner
Level 3: Add ABS
Level 4: Add Airbag
My 2006 was a Level 4 when I bought it... Airbag wasn't available yet. So if you know GoldWings all you need to know is the top level item and year to know everything that is on a Wing. But most don't know so it's better to forget about the "levels" and just list the items on the bike.
2006 added bigger radiators and fans, bigger alternator, bigger front speakers (losing the small vents under the speakers).
2006-2008 had the first NAVI version with CF Cards, 2009-2010 had a improved version with SD cards, and 2012-2017 had an even better version where you could load your own routes via a second SD card.
2012 had improved front suspension (debatable), cosmetic changes, better GPS. I believe the CD player was no longer an option and there was a USB connector for playing songs from a thumb drive (or a Crapple Ipud).
2014 had improved transmission with 4th and 5th gears having improved dogs. GoldWingGreg is a better source of info on that.
It all depends on what's important to you. They are all good in my opinion but I'd get newer just because it's newer... and would logically cost a little more.