Re: best cartridge between 300mag and 300rum
My vote goes to the 300 wby if you can accept a belted magnum or the 300 dakota for a non belted mag.
A cool idea would be to simply neck down the 338 LM without "improving" or making any other mods the case. This should yeild a bit less than the 300 RUM and you would have some outstanding brass to work with.
As far as belted mags and accuracy, there will always be a debate on the matter. My take is that if you use the intended head space for each, the beltless case will have better accuracy potential more regularly. If you hand load it is of little to no concern, it can simply be made to just be there with no use and you can set the die to just bump the shoulder allowing you to use the belted as if it were beltless.
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.