I prefer exit row if the flight is full and comfort otherwise. In comfort, you have enough leg room but still need to move for people going the toilet / ask them to move. In an exit row, the seats are narrower and the fixed arm rests mean that even if the seat next to you is free, it doesn't help you at all, but at least you have free access to the aisles which is nice during such a long flight.

When I don't know the loads beforehand, I usually book economy comfort. I find that those seats are less likely to fill up than exit rows (I guess because they are half as expensive, something I never realized since I don't pay for them)