Of course you have to consider the cost of the economy-class ticket, your interest in miles earned on UA vs. AF, cost in miles/points of the upgrade, and what else you want to use the miles/points for. There are more alternatives, I suppose, for what you can AMEX points for, so you might prefer to burn your UA miles and save AMEX for the next trip.

All else equal, I would prefer UA due to the lie-flat seat. Food and drink will be slightly better on AF, as will the IAD lounge; entertainment is certainly no worse on UA (I don't remember the AF IFE), but these are minor considerations for such a flight in my opinion.