663 has ED glass which will give you a little better resolution at high power. Larger objective will give you a few more minutes of low light glassing. I've looked through both and the difference is subtle but it's there. It's also heavier. If you are going to buy a spotter in this size/price range I would recommend the 30x fixed..it really will not disappoint. The 20-60 is fine but the 30x is a truly pleasurable view. There is not much to be gained in the 40-60 power range with a spotter unless you have a larger objective (77mm+). The image will be "closer" but the resolution no better and more eye strain. IMO to have a good view across all powers you need to spend a lot more on premium glass. That being said both of the scopes you mentioned will get the job done field judging deer and elk. If you're set on the 20-60 lens the 663 is a better choice but heavier and more expensive.Any one know the major differences between the kowa 601 and the 663? Other than 509 bucks?