Stihl MSRP says $769 for the 460, $979 for the 660, both with 20". Most dealers around here seem to stick pretty close to that price, which makes for a $210 difference.
I guess the question to ask would be this: do you need a big saw or a more powerful all-purpose saw that can act big from time to time?
I see that you're sporting a 170 and a 290, which is rather similar to my previous setup. I decided that I needed a bigger all-purpose saw more than I needed a true, dedicated big saw, and went with a saw in the 460-class with a 20" bar. I just didn't see the point in buying a heavier saw than I had to, because I knew that if the choice was between picking up some new saw with an 18lb powerhead and grabbing my 13lb 290, I would end up using the 290 most of the time while the big saw sat around waiting for the rare tree that needs a 30"+ bar. Since my whole purpose was to get a saw that would replace my 290 as my primary saw, the answer seemed pretty clear to me. Now if you do a lot of cutting with the big 30"+ bar, then you ought to go with the 660. Otherwise, save a couple pounds, a few hundred dollars, open up the 460, and just accept that you might take a few minutes more on the occasional jumbo tree.