Standard 10 twist should work, but if its the same price for the barrel/blank you can go with the 9 twist just to be safe. Only way I would think the faster twist could hurt you is if you were going to push light bullets close to their limits.
I am going to be that guy and ask: why the 257 Bob? Why not a 257-AI or better yet a 250-AI Savage? Since you are looking at a 150 yard gun the 250-AI would be more efficient, use less powder, have lower recoil, and (unless you run them real hot) much better brass with almost no trimming. 250-AI will give you the 250-Bob level of performance you want to boot.
Just some food for thought, but to your question either twist should work, but 9 definitely will get 'er done.
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