You probably should work in CDJ mode most of the time. In Vinyl mode you can only touch the outer ring without stopping the track on the CDJ-800.
If the platter of the Denon DN-S3500 is spinning you shouldn't have the problem.
For temporarily slowing down or speeding up a track, pitch bending, there are several methods on both decks:
With the pitch fader while you keep an eye on the pitch indicator on the display, this is pretty difficult method and takes a lot of practice.
By pushing the outer ring of the CDJ-800 in vinyl mode, but in cdj mode you can also use the jog
On the DN-S3500 you can use the platter in several ways to pitch bend, but you have different platter modes BEND and MAIN. In BEND mode it will be as a CDJ-800 in cdj mode. In MAIN mode it will be as a turntable platter, so you have to touch the spinning platter lightly.
with the pitch bend buttons only on the Denon DN-S3500.
In my opinion it really doesn't matter which deck you choose to work together with turntables.
The Denon has more features, but the Pioneer feels more comfortable. And it doesn't has any bug.