Re: 300 WM.. what is the farthest....
Well, if you're like me and always in search of more horsepower (those that aren't are usually perfectly happy with the 30-06) go for the RUM.
If you settle for less, eventually you'll want more--like me, I'm having my A-Bolt in 300 Win rechambered to RUM.
As Darryl mentioned, this is assuming you aren't already planning on a big, expensive custom action. In that case, the bigger the better!
But like he said, after the RUM the costs go way up. I'm having my 300 Win rechambered for about $110. Not bad for an instant 25% increase in powder capacity. The 300 Tomahawk will work in all the same actions but might be a bit more expensive depending upon your gunsmith. Mine didn't have a RUM reamer but he bought one at no cost to me since he knows he'll get a lot of use out of it--he wouldn't have done the same for the Tomahawk. If you aren't worried about that and don't mind fireforming cases, the Tomahawk should show a slight advantage. I'm thinking that's probably as much powder as you can possibly (also easily and cheaply) stuff into a standard sized action.
Hopefully Norma will start making brass soon. Let's keep pestering them! Since Federal already sells loaded ammo, I figure it's only a matter of time before they offer brass.
Accuracy-wise, I think it comes down to how much you spend on your rifle and your reloading skills. Headspacing on the shoulder is supposedly "inherently" more accurate but I'm not sure if this actually shows up until after you've spent tons on the rifle or not. Maybe somebody else knows better.