I have the I/O and I love em. Comes with spare lenses and really easy to change out. No fogging issues as most 'new' goggles shouldn't anyway. I never tried the phenoms though but I love the field of vision I have with the I/O.
I'm with Tarzanman. I've owned both and stuck with the phenoms because they didn't fog up as much as the I/Os. Both were great goggles, great fit and field of vision and easy to change lenses. You can't go wrong with either.