I own a 3913, 3913TSW (late production, middle 2000's) and a CS9 (late production with steel sights and ambi safety/decocker).
My original 3913 & CS9:
The 3913 has a bit more muzzle-forward heft and balance. The cycling is a bit less abrupt and more comfortable.
My 3913 wears Hogues (at present), but my 3913TSW wears the stock grips. The softer Hogues can help reduce the felt recoil, but I've come back around to liking the way the stock grip feels during recoil & cycling. Choose whatever you like better.
The CS9 has excellent Hogue grips that fit my hand as if made for me. The extra cushioning and thicker profile allow for very good control. The recoil impulse is brisk and a bit snappier than with the 3913/3913TSW ... but, it also allows for the muzzle/front sight to quickly snap back onto the intended target. The balance is more rearward in my hand, and what I like to call "lively" and responsive.
If someone likes to use slide weight and 'muzzle forward heft' to help them control felt recoil force, the 3913 is probably a great choice.
For someone of perhaps increased experience and skill, the slightly more abrupt recoil impulse, but faster cycling of the CS9 can make for a very controllable experience.
I've found my CS9 to be every bit as practically and inherently accurate as my 3913, in my hands. I've used it to make called hits on wooden clothespins placed around cardboard target back boards, from 5-10yds. (Easier at 5yds

).
I've used whatever issued ammo has been available, which has included standard pressure 147gr, 124gr +P and 127gr +P+ loads (I replace my recoil spring sooner when running mostly +P or +P+ loads).
I like them both.
