Can I start by saying that I have never used either of these units, but have looked at both as a possible next purchase.
I don't know how you plan to use a satnav, so that may be the big question that your going to have to ask yourself. If like me your going to stick to tarmac, then the 660 is possibly the best choice. I will stand corrected on this but I don't think the 660 is aimed at or intended for "off road" use. The Montana on the other hand is. And as such would be a great choice. My big problem with it is that if I purchase one, I would also have to buy City Navigator maps for it. That starts to bump up the price. And as much as I would love the long battery life and Topo maps that the Montana has to offer, I have to ask if it's worth it.
P.S. Out of curiosity, what were the bad point of the 660 you mention