Wow. Apples and oranges.
I have all three. 645, 4506, and SW1911. They 645 is a different gun than the 1911.
They are different in the way they operate, SA only vs. DA/SA. As TexasSTEP pointed out, the grip angle is different.
As a duty gun, I'd much prefer the 645/4506 to the 1911. For bullseye competition, the 1911 is my better choice. For IDPA type competetion, I'd lean to the 645/4506 between the two.
For open carry, I'd just use whatever I felt like at the time.
If you can get both, do so. You'll quickly see what I mean.
Between the 645 and 4506. The 645 is heavier and as such absorbs recoil quite nicely. I do like the steel backstrap with it's no-slip checkering. The 4506 uses the wrap around plastic, which works well. They make two styles. Both are easy to get from S&W. the straight back and the bulged back.
I slightly prefer the 645, but love both.
Jimmy