The new, Sport model has the 1/8 twist and is produced by Thompson Center, a subsidiary of S&W. The standard M&P 15s retain the 1/9 twist which handles all but the heaviest rounds and also the lighter, varmint type rounds.

Both have advantages but the 1/8 is better IF you are shooting the heaviest bullets available. The 1/9 is fine up to about 70grn bullets and should be slightly better for very light loads. Both handle the popular 55 grn. rounds well.