On velocity and weight of bullets, this is my experience with a 300 WSM
. With H4350 I can push 180 (E-Tip) bullets about 3000. With RL17 I can push them about 3400. I can push 210 Bergers about 2935. This is out of a 24 3/8ths Sako barrel. I was shooting my WSM once and the gut next to me was shooting his WM. He wanted me to chrono his load which was a 180 bullet and RL22 so he asked me to shoot a couple over my Prochrono. The velocity of his load from a 26" barrel was identical to my load from a 24 3/8 barrel, 3400 fps. As pointed out, the chamber of of the WSM is shorter than the WM by almost exactly .5", so the actual difference in bore was about 1".
With RL17, the 300 WSM becomes the equivalent of the 300 WM. I haven't heard of many using RL17 in the WM, but I'm guessing it would give higher velocities. With the standard powders, the WM is going to get about 100 fps or so more velocity than the WSM.
As for accuracy, my rifle is a Sako lemon and wont shoot anything well. But I don't believe there is any inherent difference in accuracy between the two cartridges and is much more dependent on barrel and rifle. Both cartridges have been used successfully in BR.
I personally don't care for belted cases because there is no reason to have a belt (unless for head spacing in the 375 H&H) and I like a clean case wall.
I think the 300 WSM is a fine cartridge and if you use a good barrel and smith you would have a fine fine shooting rifle for all around big game hunting in NA. Same could be said for the WM. Take your pick.