Rule of Thumb: If the flight is going to last 4 or more hours, fly First Class. Your body will thank you.
Yeah...if only that were global corporate travel policy for all pay-levels that are required to travel.
You nailed it.
Being 6' 3", flights longer than 5 hours are pretty unbearable in the cattle car section of the plan. My wife sprung for first class tickets for us a while ago for an anniversary present. That ruined flying forever. Now, I can't fathom sitting in the back of the plane and have a leg go numb from the inability to move it.
That being said, on some of the smaller planes, first class ain't really worth the extra money. You do get to get off the plane first as well which helps waiting all that time for people to grab their luggage from the bins.