With the imposition of copays for international upgrades on American, I've generally found that it's a better value to cash in miles for premium class directly than to get a coach seat and upgrade.

Similarly, coach is sometimes a very good deal; 40k miles + < $100 in taxes is a very bad deal when off-season London tickets are around $500 - it's starting to look like a good deal when Buenos Aires tickets run around $1000, even if you account for the lost miles, and while it's unusual to find a low-miles ticket when the fares are above par, it's not entirely unknown.

It also depends on how you balance EQM vs. RDM - in general, I earn RDM faster than I use them, and save them for "big bang" trips with my wife. OTOH, the value of keeping PLT is personally very high and having a year I can make EXP looks unlikely - so on years when I expect to make PLT easily, my goal is generall to minimize my cost of EQM up to that limit, and replacing a pricier flight or two with award tickets (as I did for one in 2009) makes a lot of sense.

Similarly, "fly this route twice, get the third free" (with PLT bonus RDM) makes a lot of sense for some folks I know.

OTOH, neither one is necessarily the best way to maximize the cash-out value of your RDM.