EY has a very good summary of the differences between AISC and Cash Costs.

http://www.ey.com/Publication/vwLUAssets/EY_-_All-in_sustaining_costs_and_all-in_costs/$FILE/ey.com_gl_cost%20per%20ounce.pdf

The gist of it is that AISC is calculated by the mining company and not the auditor. You are trusting the gold mining company to be able to present you an honest assessment of their costs. This is one of the reasons that I don't trust that metric. I would rather take the cash cost and then go into their feasibility reports and financial statements to examine future costs. More work, especially if the company has multiple mines, but its your money at stake.