I have not done head to head comparisions between the 7mm WSM and 7mm Dakota but I can tell you from the couple dozen rifles of each I have built, in same length barrels, loaded to similiar pressures, the 7mm WSM WILL NOT outperform the old 7mm Rem Mag.
It will come within 50-75 fps with same length barrels but it will not outperform it on average and the heavier the bullet you use, the more advantage the Rem Mag has over the WSM. The problem today is that the 7mm Rem Mag is down loaded so severely to make the STW and RUM look better on paper.
If you look at factory load levels, yes the WSM outperforms the old Rem Mag but when you realize that in proper handloads, the 7mm Rem Mag will do anything the 7mm Wby Mag will get you, it clearly shows you why it outperforms the smaller WSM.
Its easy to manipulate numbers to make a smaller round look more impressive then a larger one, that is basically what has happened with the WSM hype. Do not get me wrong but most of what is told about them, even by their manufacturers is hype in one way or another.
While the WSMs are close enough to the standard belted magnums to combine them in the same performance class, they will not outperform the old rounds. Its just not sexy to say "The 7mm WSM comes close to the 7mm Rem Mag".
The 7mm Dakota, its a potent chambering. Its only limitation really was that the old brass was pretty soft. It is a ballistic twin to the 7mm STW, nearly fps for fps in same length barrels and with same weight bullets. Hornady is supposed to be making the new brass and I have heard its much harder and will take more firings per case. That is good news.
Fifty driver is dead on on all accounts. The comparison and differences between the , 7 mag, 7 STW, and 7 RUM is a interesting discussion. Let me say, that right from the git go, I am a staunch supporter of the STW, and the 7 Mag as well. The 7 RUM really didn't catch on, as in anything else, you can have TOO much of a good thing, and the RUM is just that. It didnt shoot well for me, try as I may, and by the time I DID get it to shoot, the velocities weren't much over my STW's.
On average, I can get another 150-200 FPS over 7 mag speed, all else being equal, meaning bullet weight. The STW is SHOOTABLE, and accurate, and very hard hitting. The 7 mag, is a little slower, but not too slow, and recoil is very manageable. Actually, the recoil in the STW is also manageable.for extreme long range, go with the STW, and don't look back. But, the seven mag will not let you down either. TWO of the three you mentioned are excellent cartridges. IMHO.