I wanted the optional deck so that I would have a flat surface for car-camping, it was a must have. The sliding part was nice, but not as important to me.
Having had it for awhile now, I find the sliding deck is great for old guys like me because it makes a big difference on the back - much easier - to load heavy range bags onto the extended shelf and slide it in, rather than reaching over the rear bumper and into the truck to load up.
I have yet to camp in my truck, but I would definitely get it again next time. It's not like the T4R has a bunch of hidden space below the back deck [like my Prius
].