Here in Albuquerque, New Mexico many MTBers wear pads. Our trails are rocky, and even more, there are cacti all over the place looking to leave a spine in you for just passing by! I have found for a good balance of protection and comfort, I really like sleeve style pads. I wear now on every ride POC VPD Air knee and elbow pads, the sleeve kind. The elbow pads especially, I can easily feel like I am not even wearing. I had the VPD Joint Air knee pads for awhile, which were really good too, but I felt them, but nothing that distracting from riding. I only got a couple of rides in with the VPD Air sleeve style knee pad before my own bad crash tearing my rotator cuff. The other benefit of the sleeve style knee pads, is if you like me care, then no skin shows from the bottom of your shorts to your knee pads. From the waist down past the knee all covered up even when pedaling so no MTB fashion flub.
If you decide on the POC sleeves, and find yourself between sizes, get the next size up is my advice.