well, since you're asking.

I prefer the styling of the 660. The ST is just a bit too bulky to my taste. Looks too much like a Varadero as well. And those plastic tank/radiator protectors look very cheap.

The ST will be a cracking long distance tourer on paved roads, I'm sure. But all the "adventure" buzz Yamaha is selling around it is just marketing BS (pardon my French). It's actually just a beefy TDM with a shaft drive and a couple nice electronic gizmo's. But I'm sure its looks will appeal to the Ewan and Charlie fans...

Pricewise it should have been cheaper than the Beemer to really compete with it imho.