You can also put a 200 gr ULD RBBT in a 7mm STW and drive it to around 3000 fps and you will have an extremely potent long range round. This bullet has a bit higher BC then the 156 gr ULD RBBT in 25 cal. All that said, it really comes down to what you want.
Just to shed some light on the differences (although this isn't about the .257 STW) of the different rounds (7mm, .30 AX and .375 AM). If I have them wrong, please correct me. Two weeks ago I shot 3 of your rifles back-to-back on a low wind day, at a 868 yd gong. Since all 3 calibers impacted within 6" vertically/horizontally (my estimation) with the same hold, and we didn't compensate for any wind with all 3, what was the moa drop of each round if they were sighted in at the same distance?
Check your e-mail. The .257 will be headed out your way as soon as you let me know if you need anything else. Thanks!