At least with a 1:8 twist barrel he can shoot the heavier bullets, which apparently is a requirement for accuracy past say 300 yards (roughly).
I can tell you what scope to NOT get: a Sheperd PE-1. I bought one secondhand a few years ago, and have never been able to really use it. The bullet drop scale is all wrong for .223, so holdover is largely a matter of guesswork. The reticle, while innovative in concept and easy to zero, is crap in practice. It moves with every shot, which is especially noticeable at high magnification.
Other than that the Sheperd isn't too bad - no terrible chromatic aberration or obvious distortion, fairly bright. But, due to the above factors, it is totally unsuitable for slinging .223.
A lot of the Precision Tactical Rifle guys at Zia seem to favor Nightforce scopes. As previously stated - not cheap!