The only way to do this analysis correctly is to look at two factors: (1) how many miles/points am I earning per dollar spent, and (2) how much do I value each point/mile.

To make it even more complicated, you need to ask yourself what the composition of the $40,000 you are likely to spend will be, and whether there are other better cards for you to get.

But assuming you're asking a straight question about these two cards, and you are only going to put $40,000 on one or the other, your equation is pretty simple: You can either get 1 SPG point per $1 or you can get 1.625 Amex per $1. (Assuming you are not planning to spend any of the $40k at SPG properties.) Now that you know that, all you need to decide is how much you value each point. I value SPG points around 2.4 cents per point and Amex around 1.6 cents per point. So, for me, each dollar spent on the Amex would yield me 2.6 cents in value (1.6x1.625). And each dollar spent on the SPG would yield me 2.4 cents in value.

Once your values are set, it's easy. My guess, though, is that much of that $40,000 spend is spending that would earn bonuses with other cards, which to me makes it much more difficult.