I would say that it all depends on your receiver.
If you have a long action, the 7mm Rem Mag will smoke anything the 7mm WSM can do in a short action. The advantage comes in the fact that the Rem Mag already has a capacity advantage in and of itself but when you seat a heavy 7mm deep into the WSM for the short action receivers, you loose even more capacity.
Yes the WSM is more efficent with barrel length and in a shorter barrel that is more significant but it will not make a dramatic difference and in the end, the 7mm WSM will fall short of the handloaded form of the 7mm Rem Mag by at least 100 fps.
Other issues, a WSM can be made to feed acceptably well in a short action receiver, its best to start with a receiver designed specifically for this type of case design but you will still have a choppy feed cycle no matter what you do.
Compare that to the 7mm Rem Mag which will feed smooth as silk in a long action, if your looking for a repeater, there really is no comparision. For a single shot, obviously no advantage either way.
One more thing to consider, brass. The 300 WSM will likely be around forever, the 7mm WSMs future is not so secure. You could make WSM brass out of 300 WSM brass but you would have to make sure you position the shoulder location properly and then fireform the case to move the shoulder location slightly forward compared to the 300 WSM.
The 7mm Rem Mag has been around for a long time and will always be around as long as we can legally own big game rifles. On top of that, brass can be made from a host of other standard magnums but this really is a moot point as there are very few, if any standard magnums more popular then the 7mm Rem Mag.
In its handloaded form, the 7mm Rem Mag is the full equal to the 7mm Wby Mag. Only in factory form is this chambering downloaded severely and this is in my opinion simiply to try to make the larger 7mm magnums look more impressive. Sad really, because it really puts a poor light on what the 7mm Rem Mag really is.
Saying the 7mm WSM is the full equal to the 7mm Rem Mag is simply not true. All things being equal, it will easily fall well short of the handloaded 7mm Rem Mag loaded to standard modern magnum pressures that the WSM is currently loaded to.
It feeds better as well.
Yes it has a useless belt but after the cases are fired once, that becomes a moot point as well as the case will then headspace off the shoulder and not the case belt as long as the case is resized properly.
The myths out there about the belts causing rough feeding are simply that, just some writer looking for something to complain about when there is really nothing there at all to complain about just in an attempt to promote a new case design.
Do not get me wrong, I do like the WSM cases but in reality, they are not the full equal to the standard belted magnums that we have been force feed to believe over the last several years.
They have great performance and its always great to have as many options as possible but for my money, for a big game rifle, I would lean toward the 7mm Rem Mag. For a single shot long range match rifle, either would do but the WSM may get the node.
The 7mm Rem Mag is not sexy any more but that does not take away from the fact its an extremely potent chambering that will out perform the WSM 7mm any day of the week.
The 280 AI and the 7mm WSM are basically ballistic twins in every way, again, the AI will fall around 100 fps or a bit more short of the Rem Mag. Its a great chambering as well but it does have its issues also. Have to fireform brass unless you buy the far over priced Nosler brass. You can form brass from other easy to get cases but the neck lengths will tend to be a bit off, not a huge issue.
As far as barrel life, the 280 AI is probably the best followed very closely by the WSM and then Rem Mag but they are all very good on barrels for their performance levels.
Again, all comes back to what receiver you want to use. Long action, I would go with the Rem Mag simply for performance.