906 or 915 ?

I would have either set checked over. Unless the 915's are a deal (like $500-600) and check out good (don't need any work) I would price out what it will take to get the 906 heads ready to run.

You may find it cheaper to buy a set of 440 source heads and be farther toward your hp goal. Even if you go that route have them checked out.Don't just assume they are ready to run.