I have both the Vixskin Maverick and the O2 Max -
The Vixen has a more realistic shaft/head design, and the texturing down the shaft is very nice. The O2 Max is a bit more abstract/stylized. The sizes are very similar, but the O2 has the hard core a bit off center, though it isn't super noticeable during use. I guess they do this because in a penis the bottom has more "soft" than the top (?)
The Maverick comes with a base that serves as a suction cup, but you do need to have an absolutely smooth surface to stick it to for it to work. The O2 Max base doesn't "suck," but it is a bit easier to hold. The Maverick has a curve for better G/prostate stimulation, and the second "bump" is nice, but the very noticeable bulge at the "corona" can be uncomfortable for some people (especially if it catches on the pubic bone), and I have noticed a trace of blood if I use it more roughly anally. The O2 Max's more streamlined head doesn't have this problem.