anti vibe far nicer on 661. no longer feel it in my elbows from running the 660. but my joints are extremely flexible. So may not be the case for everyone
gas mileage probably 20-25% better with 661
661 has a lot more torque
there are pluses and minuses with each saw. And I guess I do wish I hadn't traded my last 660R away. Some days I wish I was still tuning it with the little orange screwdriver and others I like the 661 more.
661 is one hell of a saw. Stihl made a lot of improvements over the 660. But I still like the 660R. 660R is the way to go if your gonna do it, large clutch cover, wrap handle, nice spikes, and most importantly it has the high output oiler. 661 has the same oiler if it's 661 or 661R.