The difference between the two is on what's on the inside. The red is a bit less intense than the black one, as it's got little nubs, ridges that stroke the member as you use it. The black variant, has more nubs and ridges, as well has everything up higher inside of it so it actually catches one's member as you use it. Take a peak at both versions when you look at the item's main page. Personally, I have both, and prefer the black one since I'm a bit desensitized. ^-^; Both of them have the suction feature, and both feel different when you turn it to different directions on the shaft.

Also be sure to check out the reviews for each one and compare, you might learn something for them toward your purchase!