A very difficult decision and one that I have struggled with as well. I am a fanatic .338 Win. shooter and currently own five excellent rifles so chambered, however, I do not have a .35 bore. Yet, for big game hunting, I prefer the .338-06 over the .35W, I'm kinda an Elmer fan.
But, if you are going to want to whack Grouse and shoot Gophers, the use of pistol bullets in the .35 W makes it the better choice, IMO. You can buy specialty bullets up to 310 grs., I believe, for the Whelen and 300 grs. for the .338 bore. These are Woodleigh and Smithrite Bonded from Vancouver, B.C. respectively. I prefer Nosler Partition Gold 250s in the .338 and either 280 gr. Swift A-Frames or Northfork 270s in the .358 bore.
Actually, these along with the 9.3x62 are just about the finest "one gun" cartridges for the guy who hunts Deer, Moose. Elk and especially back or horsepacks in Grizzly country. You have excellent stopping power, more shots and a slightly lighter rifle than the magnums..............the only real answer is to get one of each!