I have had all the cartridges you mention plus a few more in .277". To start with you are correct in that thr .270 Winchester is hard to beat. It is a pretty efficient cartridge to start with. With a good set of dies and more importantly a good straight loading press, the .270 Winchester is an easy to load cartridge. It does suffer from the fact that the cases tend to stretch, and this is why I initially changed to the .270 AI. With the AI chambering, I could gain about 75-100 fps from barrels of the same length from the same manufacturer with modern powders (H-1000, Retumbo, R-25)and 150 grain bullets. To be honest, I have found the .270 AI to be even easier to load for, even though the performance increase is slight.
I have owned two .270 WSM barrels, and liked them both. If you have the option to pull the bullet a bit farther out of the case, the .270 WSM will really sing!!! Accuracy is very good and you can beat the .270 Win. by 100-150 fps with a few loads pushing 200 fps better. With the 150 grain bullets it is really important to be able to pull the bullets out further to reach these velocities. The big disadvantage to the WSM is the big drop in efficiency. You are burning a good deal more powder to gain a couply hundred fps. Barrel life is also considerably shorter. I noticed in one barrel that I had to seat the bullets 0.095" further out to "kiss" the lands after 500 rounds. This barrel is still in use and has 725 rounds through it and is still 3/4 MOA.
My latest build is a .270 Gibbs and initial testing is looking very promising. As soon as I get my pressure testing equipment set up again, I will release some load data. Mr. Gibbs must have been on to something because this front ignition stuff really drives the efficiency up. The big disadvantage to the design is that it takes about 30 min. to reload one cartridge......
In short, I do not think you can go wrong with any of the cartridges you mention.
John