I own both. I think I prefer the stonewash, but there is something that really sticks out with the working finish. The working finish has a look all to itself and makes a great looking knife.
I'd suggest if you own one XM 18, make it stonewash. If you buy two, make the second a working finish, or vice versa.
You can't go wrong with either one mind you, pick the one the you like best.