A Pros/Cons/Neutrals list might help you. I can only speak for 6.5 ft Screws, not Scabs. Some points may not apply to you.

Pros of 6.5 Bed Screws:
*More cubic feet of storage (duh). If hauling luggage/regular cardboard boxes/bags of mulch/etc, that extra ft. really makes a difference.
*Smoother overall ride quality with 157” WB. Mine rides like an old Lincoln Towncar.
*If you load ATV’s or a motorcycle in your bed, can maybe do it with tailgate up.
*If you add a narrow fuel transfer tank to the bed (not an aux. fuel tank like is done on the F250 diesels but a plain transfer tank for gas that allows you to refuel mowers, etc), you’ll still have ~5.5 ft of bed storage space.
*Optional 36 gal. fuel tank – when I bought mine, even though 6.5 bed Screws were very hard to find on dealer lots, of the few I found a high percentage of them had the 36 gal. tanks.
*Resale – you might get a little more because they are relatively rare compared to 5.5 ft bed Screws

Cons of 6.5 Bed Screws:
*Wider turning radius. I found a noticeable difference between 5.5 and 6.5 ft bed Screws I test-drove.
*Good luck parking it in your home garage; if you park outside then a non-issue.
*Parking in multi-story garages – you’ll become adept at backing into spaces vs. pulling in head-on.
*With 157” WB, a 2-piece drive axle. May not be or become a problem but some posters have noted issues (clunking when on/off gas pedal, etc). I’ve not experienced any problem like that to date, but some have mentioned it.
*Very hard to find on dealer lots, most people seem to order them. I did a sample check last year for grins on how many 6.5 ft Screws were on dealer lots in major metro areas (LA, Dallas, Houston, Chicago, etc) – only around ~3% of Screws of any trim level were 6.5 ft bed.

Neutral:
*Choice/flexibility of bed accessories (bedcovers, bedmats, etc). Base model Reg cabs and Scabs have 6.5 ft beds, aftermarket geared for these. But a whole lot of 5.5 bed Screws out there too with many choices. So probably a toss-up.
*Hauling long lumber/plywood sheets – as Ricktwuhk has mentioned, bed extenders are pretty cheap and easy to fit – can find them at HF or Northern Tool for under $130. So a 5.5 ft bed becomes a non-issue for this.
*Depth of bed – I was disappointed that the 2015+ F150’s “lost” about 1” of bed depth from previous 2009-2014 F150’s. If you have a hard or soft tonneau, sometimes that 1” becomes a problem if hauling pressure washers, pumps, etc. My Honda pressure washer that fit in my previous ’09 F150 reg cab with closed tonneau does not fit in my ’16 bed with new soft tonneau. Guess they had to save some $ on aluminum….

 

I had 2010 Scab 6.5 box. Great truck but wanted more cab room. I now have a 2015 screw 6.5 and a 2017 screw 6.5 and I would and will buy another. I live in a city, drive hwy and work in the oilfield... and while they are longer its not a show stopper. Maybe its because its all i drive now but I am used to it. All the rest is positive. Better ride, more stable towing, better in snowy hwy conditions (Canada), and all the room you need. To me if you're paying 45-60k for a truck, why not get all the truck you can.
As for looks,IMO they dont look great stock. I have 4" lifts going in both as we speak but my 15 has been leveled since new and that helps. Picture below is the 17 stock. and as for screw vs scab, the screw all the way...no competition. even if you don't haul ppl, the room you have with the rear seats up and flat floor is equal to the back of a Suburban!
For me, Screw 6.5 all the way.

 

I had to pick up some longer lengths of trim for the kitchen few months ago. I just fished the box in through the rear slider and up to the front seat floor. Closed the window behind it. cab is longer than the box it seems on the crew 5.5