I emailed Vacutech with the same question and got this response:
"The ThickWall
cylinder is meant for only length, and is usually sized so there is no room for thickness growth in the
cylinder. If you want more thickness you should stay with the Flared
cylinder. The ThickWall
cylinder goes deeper into the body helping with the stretch of the suspensory ligament providing more length. Like the Flared
cylinders you still need to take a break every ten minutes and give yourself a massage or jelqing. When working on length I would recommend extra time for the massage to help the ligament with the stretch."
So basically, (from what I gather) if you were pumping for length,you would "pack" the flared tube and then switch over to the ThickWall
cylinder. From the picture on their website (see attachment) the ThickWall lacks the big flared end of what I've used in the past. I guess it cuts thru the fat pad more and pulls more "internal penis" out. It sounds painful though and doesn't seem like you could pump for very long like this. That's why I was looking for feedback.
Pain resulting in gain is worth it. Pain with no gain? forget it.