It all depends on what weight of bullets you want to use. 1/9 is pretty much guaranteed to work with bullets in the 63-65 grain range (with weights below being a better match), 69s might work, and 77s likely won't.
A 1/7 twist barrel will work acceptably well with all current .224 bullets, so it's the most versatile option.
If you don't plan to sling the heavies, then a 1/9 will work, but If I were choosing a new barrel, it would definitely be 1/7.
