I've Repaired/Rebuilt Them Both And...
...both have their place. Loggers who drop lots of trees will take the MS660 first. It can pull a 36" bar and not wear you out while trying to drop a big, awkward tree. Much more control and easier too handle, regardless of how strong that you are.
The 084 is a great big, heavy bucking saw that loves to eat wood like a big dog loves to eat food. Few saws can outbuck it because it has the horsepower and over 30 more CCs than a 660. I love taking the 084 to GTGs and watching it kick the butts of 066s, MS660s, and even stock MS880s because it breathes easier. In short, it's a log bucking beast.
However, if I had to earn my living by dropping lots of trees AND bucking big logs, I would buy the MS660. The price you found for these saws is unbelievably low, even if neither runs. 'Nuff said.