I think its a no brainer from the perspective that you're fairly likely to find sales/deals to purchase a J ticket, whereas this is quite unlikely in F (esp. ex USA). Of course it depends on your travel patterns- are you flying long haul every month and just want to use as much as possible miles to get from A to B, or if you're like me flying long haul perhaps 4-5 times a year, then when I do fly I want it to be in F. Not because I can't sleep in a J seat, but because when I have limited opportunities I'd rather enjoy the trip. If i was doing it every week I'd probably have to fly Y...