Easier to get brass. Also read it was more effiecient & easier on throats according to the article on 6mmbr.
If you prefer other brands of brass then yes there are more choices for 300WSM brass.
As to being easier on throats that would only be because it's slower than the 7WSM. There is no magic in the 300WSM case which is lost when the shoulder is pushed forward .038".
I take it you don't have a 7WSM. I have found that the W-W 7WSM brass is terrible. It is a PITA to create 7-300 WSM brass compared to just shooting and reloading.
I would MUCH prefer to have a 7-300WSM than a standard 7WSM. As far as neck turning- a standard 7WSM neck is big enough that the 300 brass necked down does not need to be turned. Make sure your chamber is accomdating. As the person who will chamber it how big the neck is, measure a loaded round and make sure you have .004" total clearance or more and it won't need to be neck turned. Less than that- no guarantees.
