For me, depends on the aircraft and the seat. In general, though, if the upgrade is free, it's pretty unusual to get a premium-class seat that doesn't beat all the coach seats on that plane, even an empty 3-across row.

With miles+copay upgrades on AA international, and "stickers" for domestic (most years), I'll often stick with Y if the load is light and I expect to keep an empty seat, especially for day flights when I'm not trying to sleep.

For night flights, the extra recline -- even in aircraft with less-good F seats (e.g. the domestic 757 and older 737s, in AA's case) are going to give me a better chance to sleep than anything short of a row to myself in coach.