Take the 6.5 with the Partition...
That's what I'd take. Especially since the NP is very accurate in that rifle. My .260 with 140 gr Corelokts did a DRT, bang/flop elk at 150 - my only elk kill. Spine shot in the neck.
On the other hand, I'm using an '06 with Hornady Superperformance 150 SST on caribou out to 500 yards, mostly over 300. That round at about 3100 MV, seems designed for 300 yards and out, where the velocity slows down some. Definately ill-advised for "shoulder shots" or any major bone/muscle mass hits except CNS! The GMX (mono) is supposed to perform ballistically identically with the SST in the same bullet weight with less ("explosive" some call it) expansion at closer ranges. So just swap from SST to GMX if that worries one. Sight in/component use if handloading should remain unchanged, if the hype is to be believed. I just bought a box to see...
The only SST I have ever recovered from nearly a dozen kills was from a head shot moose at 30 yards. Nicely mushroomed. jacket with maybe 1/3 of the lead left a half inch away, seperated.
All others (caribou) to 433 yards were pass-through , behind the shoulder shots except two. One was spine shot at about 150 yards. Nothing left of that SST or about 6 inches of spine and backstrap.
Of the last 3 caribou, the first two were shot less than a minute apart at 290 and 433 yards respectively. Both thru the lungs, both rounds missed ribs both sides. Quarter sized entry, 50 cent exit. I "lost" 4 inches length of rib meat, 3 ribs wide is all, both sides. Maybe 4 ounces per animal, of burger meat.
The last one I killed, I f'ked up and touched the round off while still positioning the rifle. At nearly 300 yards the SST left softball sized holes through both hams. Operative word is through.... including breaking/shattering the femur- I forget which side.
I'd say SST's perform exactly as intended, the only questions being one's own preferences and bullet placement. If you take the '06, using the GMX would be my recommendation. But properly placed, broadside just behind the shoulder, the SST will definately do the job! If it hits a rib going in, the wound channel will of course be greater. If you are one to take Texas heart shots, or shoulder shots, do NOT use the SST, especially under 300 yards!!
Personally, I've largely gone back to c&c bullets, Corelokts and Interlocks primarily, as for my use, they do anything (often better in accuracy!) a premium does , and i'm cheap.....
The only bullet failure I've ever had, was, interestingly, a factory loaded .338WM 210 NP, on the shoulder of a moose at 100 yards or so. BB sized bits of lead and bone peppered the nearside lung, and nothing reached the far lung. Never did find the base - i think it fell or richochetted back out the rather large entry wound. The second shot virtually up his nose at less than 10 feet when he lurched back up was a bit exciting, and worked. One CAN shoot a RU77 at full arm extension, one handed, provided sufficient motivation. John Wayne lives!
Kinda soured me on that bullet, which I never used again. Didn't do that much for my opinion of NP either, tho I have used them in 250 in that rifle with good results, including fair accuracy. And in other rifles and bullet weights. They seem to be something beyond just a passing fad.......