Backing up a TT is easier. They respond quicker and you don't need to worry about swinging the front of the truck around as much. For towing a fifth wheel is better, there's just no getting around the fact that your setup will be more stable when the actual hitch point is over the rear axle. I think the extra pin weight helps too, but the real key is not being hitched up 4' or 5' behind the rear axle where the TT can apply leverage.
Aside from those points, a FW gives you more trailer in a shorter overall combined package but does so at the expense of space in the truck bed. A FW is also generally taller and has interior steps to deal with, which are negatives to some.