The 1:8 is a compromise between 1:7 and 1:9. Keep in mind that what you gain in making the bullet spin faster, you are losing in forward speed. There is only so much energy in the propellant and if more of it is going into the rotational energy of the bullet, there is less to go into the translation energy.
I'd always been curious about this idea, so I did some informal testing a few summers back using my handloads and 20" AR uppers with different twists. Though the physics of it makes me believe I should see a loss in velocity with the faster twist, the chronograph showed no appreciable differences between the different twists. Perhaps the differences would have been more pronounced in a larger caliber, shorter barrel length, etc; like is said, it was informal testing and by no means comprehensive. But what results I did get leads me to wonder how much velocity difference twist really makes overall. Perhaps someone else can add to the subject, becuase it does have me curious.
Regarding the OP's question, it's worth mentioning that there are other factors in bullet stabilization besides twist. Length of barrel, velocity of round, and shape of bullet all influence stabilization as well. So as a for example, I've found that the Hornady 75 BTHP will stabilize just fine and shoot quite accurately in my 1:9 twist barrels, but that the Hornady 75 Amax will not. The bullets weigh the same, but when you compare the two you see that they are shaped differently so that one will engage the rifling and stabilize and the other will not in a 1:9. I've also been able to get some of the lighter weight bullets to stabilize in faster twist barrels by backing my loads down so they're not going as quickly, and found at times I can push heavier bullets a little faster and get them to stabilize in a twist that might be too slow.
So point is, while twist is the primary influence on bullet stabilization, there are other factors at play as well. Generally speaking though. I'd agree with Kraig that you're best to get the faster twist barrels unless you are looking for something more specifically to shoot lightweight varmint style bullets.