I rebuilt an MS-660 and a 288 side-by-side last year - both completely stock, as they would be from the factory. With a 32" bar on each and brand new 36RSLK
chain in a 30" fresh Doug Fir log, I couldn't really tell much difference in cutting speed between the two.
What you get with the 288: Better anti-vibe, usually a freer-flowing muffler, better carburetor, stronger crank, better oiler, and a few ounces lighter.
What you get with the 660: Bullet-proof ignition, takes more abuse, crankcase is a better heat-sink, better air filtration (unless you get a 288 with the HD system),
inboard clutch, a little more long bar torque. I prefer the handling of the 660 as well, but I ran both in the woods for years professionally.
Both take mods well - the single-ring piston in the 288 can be detriment for long-term heavy use like stumping or milling. Both have a consistent history in hotsaw racing.