So, here's the problem. It's not so much the barrel as the rifle platform itself. If you want to feed from a magazine, you can only have the overall cartridge length up to 2.26"
With a 1-7" twist you can stabilize heavier/longer bullets, but you run out of room to feed them through a magazine. But, you can single feed them, if you work up a load with the bullet to or into the lands. With a standard 5.56 throat, that gives you an extra .120" Long enough for 90 SMK's. (the whole premise being to not load a bullet below the ogive in the case.
For feeding with a magazine, a 1-8" will stabilize the 75 and 77 gr. HP's which will get you to 1k.
Bottom line is if the latter is all you want to do, no problem get the 1-8" if it's less expensive. The 1-7" twist will give you more lattitude with heavier/longer bullets. And, you can probably find a good one for the same price as a 1-8".
Also, a long time ago, we were not able to get the concentric standard of manufacturing we see in bullets today. Therefore, we always used to use only as tight a twist as what was required to stabilize bullets. Less of a twist left less of a harmonic imbalance in how the bullet remains stable/accurate in flight. Old time benchresters still live by this and you will see a lot of 1-14" twist .22 cal and 1-13" twist 6mm bench rest barrels. With the higher capability of todays manufacturing in bullets, we can get extremely good accuracy in 1-7" and 1-8" twist using light bench rest bullets.