Rebuild 915 heads or go aluminum?

The costs are reasonably close, but you will spend more than iron to put aluminum on it. Nothing is good enough for me to run out of the box. The lesser lines of heads need parts and the valve job fixed. The RPMs just need the valve job fixed and the springs replaced if the cam warrants it. The cheapest aluminum will run you about 20% more than well done iron.
If it was me, I'd leave the heads you have. If it needs a little more steam stick some bolt ons into it until you can build what you want. Just me though.