Re: Any advantages of the 300/7mm wsm?
Colin - the question probably breaks down to which slug seats well - you may need to choose the option with the longer neck, as that alone could make the difference in accuracy down range. I'd probably start out selecting the slug with the highest BC I could find in that caliber and see how well it seats in the shorter neck before building the gun.
I ran some math on the chambering differences. I sincerely doubt the miniscule powder charge increase is a noticeable advantage. You could possibly cut the deeper chamber and keep two bolts of different lengths to see which performs better.
I DO NOT RECOMMEND THIS PRACTICE TO ANYONE, as mistakenly choosing the long bolt or the with the long bullet or the short bolt with the short bullet could cause a disaster, but it is a way a VERY CAREFUL fellow could check out both cases with one gun. If I was that curious myself, I would paint one bolt red and one bolt green, taking only one bolt and one (the appropriate one) set of ammo to the range on any one day.
If you are making a claw extractor, you will likely need to make one for each bolt, a red one and a green one. If you do try this, PLEASE don't get blown up! A world record group ain't worth it! You're probably better off building both guns.
JeffVN's point is very good - you could long-neck the chamber and use the 270WSM brass and never really regret anything.