Sounds like it is too late, but for future readers with the same question, I'll toss in my two cents worth.
I looked at the 660 and 665. The screen brightness was an issue. I also felt at my advanced age (over 60), and after using both 4.3 inch and 5.0 inch car GPS's, I wanted the larger format.
I picked up a refurbished 590LM from the Factory Outlet Store. Price was around $550, which I thought was steep, but when you look for a "motorcycle" GPS, you're stuck on the bad side of the supply and demand curve. Still, it is cheaper than a "new" unit.
Everything about the 590LM appears new. The box. The unit itself. It even has the full Garmin warranty.
The screen is fine in bright sunlight. Navigation includes a curvy road option. I simply set up two profiles. One as a "car" for fastest routing. The other as "motorcycle" to take advantage of the curvy road option. Traffic updates are good with a smartphone app tie-in. So far, in over 6000 miles of mostly winter Seattle riding, it has worked fine.
If given the choice of the 660 and the 590, I'd go with the 590 for the larger screen and curvy road feature.
Chris
Chris