The grip is a little fatter than the 645. I've owned both pistols and like the ergonomics of both. On the 659, the grip is also a bit shorter front to rear than the 645. Also, if the 659 proves a bit difficult to find, the 5906 is a similar pistol with a single factory grip that also takes the place of the backstrap that the 645 has. There are some that have become available on the market through several sources. Here is one.
Smith & Wesson 5906 | Birmingham Pistol Wholesale