Justin,
What do you want to do with the rifle you are chambering in one of these choices?
These are very different critters, although all 257 caliber, they are basically in different catagories as far as use and performance.
The 257 Wby Mag is by far the most user friendly, also the easiest to load for as it is the least extreme. This chambering is at its best with bullets in the 100-120 gr weight range.
The 257 STW is a dramatic step up in performance but also limits the usible powders for such a large capacity round. If your using bullets of 110 gr and heavier it is a ***** cat to load for with extreme performance. With a 100 gr pill this round will do just over 3900 fps in a 30" barrel, 3850 or so in a 28" barrel.
It will also drive a 130 gr bullet to 3350 to 3450 fps in a 28" or longer barrel.
This round is best suited for bullets in the 115 to 145 gr weights. The 100 gr pill can be used but it is really to light for this big case, the heavier bullets simply work better.
The 257 AM is a monster even compared to the 257 STW. As the designer of the Allen Magnums I am also the first to tell you the weaknesses of these wildcats. Ballistically there are not many except finding a bullet that will survive the severe impact velocities produced by this round, even at extreme range..
Like the 257 STW, the Allen Mag can be loaded with the 100 gr bullets and will top out in the 4000-4100 fps range with a 28" or 30" barrel. Still with such a huge case capacity, these light bullets can be erratic in velocity.
This rounds really starts to run where the STW drops off in bullet weight, starting with the big 130 gr bullets. From here up to the huge 156 gr ULD RBBT is where the 257 Allen Magnum runs extremely hard at long range. With this bullet weight and the velocity potential of this round, it places the 257 AM right there with the largest conventional 7mm magnums on the market. Well, to be honest it is ballistically far superior to them but it is still a 25 cal round which is the reasoning for my comment.
Still, it is an extremely specialized round which is really only suitable for the VERY experienced hand loader. One can only use ball powders, luckily there are a couple that work perfectly that are easy to get. The 257 AM provided legit 1000 yard reach on deer size game but it does have its issues.
It really depends on your goals for the rifle as to which is best for your purpose.
Let us know that and we can give a better recommendation. I am the first to recommend against my own round if it is not the right chambering for what your looking to do.
If your looking for an easy loading high performance deer round, the 257 Wby would be my choice. If you want flatter shooting with still relatively easy loading, I would go with the STW.
If your looking for the ultimate in performance in a 257 chambering, the 257 AM is the tops but it is also the most extreme of all the Allen Magnums and as such the most sensitive to load variation. If you think you know alot about advanced loading of wildcats, the 257 AM may be for you. It will teach you things you never though you would ever need to learn. I say this from personal experience.
There is a reason there are not more 257 AMs out there. She is one wild ride to work with. Once a good load is worked up though, its truely an impressive long range round which makes all the other 257 magnums pale in comparision to her.
Kirby Allen(50)