There is nothing wrong with the RCBS dies that you have been using. If you just want a change, decide how advanced you want to go. You can do neck bushing dies, FL dies with a neck bushing, hand dies with an arbor press. The list goes on and on. Personally, I either use the Redding competition dies or Wilson dies on my custom rifles and the Redding Deluxe die set on my factory rifles. The Redding Deluxe set is just the standard FL set with a neck sizer included. Its a little cheaper as a set than buying a neck die later.
My advice for loading a 300 WM, or any other belted magnum, is to set your sizing die to size the case to headspace on the shoulder, not on the belt. The cartridge is not hard to load for, just don't let the belt confuse you.
There is really no wrong choice for choosing a brand of die. Redding and Hornady are favorites her on "The Hide". They all work. Lightman