I've owned and shot a 7mm-300 WSM since late 2001 when Winchester released the 300 WSM. Like everyone else, it was an opportunity to neck it down and neck it up to try alternatives on the same case. Everyone looks for that golden chalice of perfection.
We even chambered several rifles for the then newly released 7mm WSM with the shorter neck. Performance was very nearly the same when you take into account that the barrels all might be slightly different. But, and here's the big difference:
It is slightly easier to control the neck tension and the concentricity on the 300 WSM case
AND currently there are better options for brass selection when using the 300 WSM. The 7mm WSM cases can be difficult to source even with the huge number of online retailers selling brass. Winchester has become stingy with the release of brass over the last few years.
I happen to prefer the 7mm-300 WSM for the brass selection. The shorter neck on the 7mm WSM didn't really bother me as I have seen examples of shorter necks being extremely accurate:
For hunting, I've standardized on the Berger 180 gr. Hunter VLD over a maximum load of Re-25. This is an extremely accurate combination in my rifles and is suitable for hunting elk if that opportunity presents itself to the members. This load is comparable to the information found on Hodgdon's reloading website although I'm running just a tad faster...
I forgot to add this thread:
https://www.longrangehunting.com/threads/7wsm-or-7mm-300wsm.44947/
And this shows the difference in neck length:
Regards.