Would just like to make one correction to your post. The actual BC of the 142 gr ULD RBBT in 25 cal is more on lines with 0.700.
With this number the two Allen Mags, 257 and 270 loaded with the 142 and 169.5 gr ULD RBBTs are pretty much ballistic twins.
It is true the 156 gr ULD RBBT has a higher BC then the 169.5 gr ULD RBBT 270 bullet but in my testing it will top out around 3300 fps with a comfortable load compared to nearly 3400 fps or a bit more in some cases with the 270 AM.
This is all besides the point anyway, I do not recommend either one of these for any type of varmint hunting use. They can certainly be used for that but the user must realize that barrel life is limited anyway and when used in such a role either will be very hard on a barrel.
As a big game rifle they will last many many seasons and in most cases a lifetime of serious big game hunting.
They are both extreme performance rounds. The 270 has the dramatic advantage in user friendliness as far as case forming and loading. The 257 AM is on the very top of the 25 cal performance pile and as such is a very extreme and sometimes finicky round with special limitations in powders that can be used to load her.
Just wanted to offer my opinion between the comparision.
Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.
No problem Wildcat, FiftyDriver knows more about it than I do, and I was wrong about the BC of the 142 ULD...
FiftyDriver says he has gotten 3500 fps from a 1:7 with the 142 ULD in the 257, but I would be the 9 twist would do much better....
With the 257 AM you are stuck with Ball powders, they can be somewhat temp sensitive.. meaning.... you might lose 200 fps from 60 deg to 20 deg.. I am just throwing a number... but I have heard they are temp sensitive in velocity... at 700-800 yards, it could make a differance...
Still a .700 BC bullet at 3500-3600 fps isn't going to drift much at real long range... Doping with the wind can be tricky...
Fifty is right, not a good Varmit gun, if the varmits are prarrie dogs or woodchucks, but Varmits like coyotes that are not quite as plentiful... maybe a good gun
Where I live, it would make a good Yote gun... We maybe see 2-3 Coyotes a year, heck I haven't shot at once in over 2 years...
my next gun will either be a 257 AM or probably a 6mm284 to shoot 105's for coyotes at range.