I've had both and used them both without a case.. the jet black is much, much grippier in my opinion. I never feel like I'm going to drop it despite my small hands and how big the phone is. Loving it!!
I had a matte black and would never have considered using it without a case, as it was very slippery. Although I actually prefer the cool to the touch feel of the matte black, the jet black is far more "grippable" and I am perfectly willing to go caseless. In fact, that was the primary reason I swapped my matte for JB.
If your hands are slightly moist the jet black is like the grip on a glass window but if it's real cold they both are like ice. With those rounded edges they both are horrid. iPhone wasn't made for human hands get a case