I would go with the 30 caliber. For me long range shooting is out to 600 yards. If you want to shoot the heavier bullet's then go with the 300 WM. If 180 or less then go with 300 WSM.
As far as your belt question. When the WSM first came out it was touted as a more accurate round-over the 300 WM. One of the reasons is the lack of belt. With a belt-the cartridge headspaces off the belt. Without the belt it would headspace off the shoulder, which is suppose to be inherently more accurate. You can headspace a belted magnum off the shoulder, but it is much more difficult.
Below is a picture of my newest 300 WSM. I shoot 155 grain Palma's at 3250(H414 is a great powder). It shoots under 4 inch groups at 600 yards. I will be using it for deer this year. I will limit my shots to about 625 yards. Maybe next year I will be able to extend my range some more, but right now anything over 600 and I have some doubt. Out to 600-LOOK OUT. Tom.