Where the 300 WSM will shine is running the wonderful 200-gn Accubond (BC .588) at close to 2900 fps. There you've got a great setup from 7 yards to 700 yards.
BUT, aside from the advantages of the higher SD of the 200 NAB over the 165 NAB on closer shots (the 200 will out-penetrate on average), the 200 from the wizzum won't show a meaningful advantage until you get out past 400 yds or so, and/or in windy conditions. At that point the 200 will thoroughly trounce the 165 (I shoot both, from an '06 and a 300 WM).
The 200 NAB from the WSM is better in every way, close to far, but does it actually matter? Not on a broadside elk, and not under 400-ish yards. If you are desiring the ability to take tougher-angled shots that broadside and/or shooting longer than 400 I'd go WSM/200 Accubond all the way.
IMHO.
