I've had both. My last truck was an F250 SB with a slider. Before I got my current truck, a RAM 3500 LB, I did extensive modeling with a CAD program to determine how tight I could turn with a short bed without pulling the slide. It was approximately 12 degrees less than what I could get with a long bed. Of course, with a short bed slid back it would do a full 90. Do you need that extra 12 degrees? Maybe not normally, but it's nice not to have to worry about a tight turn in a gas station, or in a tight campground spot without having to get out and slide the hitch. I opted for the long bed.
Now that I've had a long bed, I'm glad I went that direction. I don't have to worry about sliding the hitch back at all, and since I've had the long bed Ive loaded it up with lots of stuff.
The only downside of a long bed is having to park when you are not towing. A small price to pay.