^
Actually, I'd suggest sw for the lower levels. I'm pretty sure cannon damage is partially influenced by your ranged level, and so using it below 40 ranged would probably be a large waste of money.
Agreeing with tyr0n3 on this one, you're better off using the Ranged Guild but you said you have 20 M. at your disposal right? Multi-cannon your Ranged up to 40, it's definitely going to be faster that Soul Wars for the first 40 levels... then you'll have the awesome option of using the Ranged Guild! or if you're not a fan of that there's the much slower option of Soul Wars. This truly comes down to comfort rather than speed though, especially for such a tedious task. The third option that nobody seems to be bringing up however is calculating how many cannonballs it would cost you from 1-99, 40-99 or whatever level to 99 Hope this helps!