Fox Mike said:
What does the .44 special offer that the .45 Colt can't do?
The name: ".44 Special" (and Skeeter Skelton)! We oldsters read Skeeter's championing of the 44S in Shooting Times 30-40 years ago until he passed. It's truly a good cartridge and my favorite, but let's be objective, there's a lot more substantive answer than that and you asked the right question: What does it offer over the .45?
Another good question is: What does the .45 offer over the .44?
.44 Spl:
It was designed for smokeless powder with a compatible case size for good volumetric efficiency. It can be loaded up or down and has inherent accuracy in most all loadings from low power to heavy loads.
It's relatively a fairly flat shooter and accurate at all different ranges.
It has good knockdown power.
Works well in medium frame sizes for packing around.
.45 Colt:
Who doesn't like saying they shoot a .45 Colt?
The Colt was a black powder cartridge which gives it a larger case than needed for smokeless powder.
It can be loaded for excellent accurately as well. But at lower loadings the accuracy drops off as well as it's consistency from shot to shot because of its overly large case volume.
Trajectory is like throwing a softball and point of impact varies more over different ranges.
However, also due to its large case capacity, it can be loaded to true magnum power levels, performance, and flatter trajectory. But to realize that performance (so called 'Ruger Only' loads) you have to use it in a bigger gun.
It's sectional density is greater than the .44 with great knockdown power.
I won't say the .44S is better than .45 Colt, or vice versa, they both have their virtues. So it all comes down to what you want to do with a cartridge and like most things in life, personal preference.
Solution: get both!
Hope that helps,