I have the ER6i's and have listened to my friend's Vibes. They're about as different as you can get, yet each good in its own way.
The Vibes are small and made of metal; the Etys are really small and made of plastic ... the Ety's must be no larger than half the size of the E2's (unscientific guess).
Some of the Vibes have a particularly cool braided cord within a clear casing; the Etys have really thin cords (some people don't like the microphonics, but I usually use mine while sitting still, so not an issue for me).
The Vibes insert into the ear canal but don't really block out outside noise, although there is essentially no sound leakage outward; the Etys can use a variety of tips, and if inserted correctly can serve as reasonably effective earplugs even with no music playing (I've tried that successfully on airplanes). On my Etys I use short Comply foams for comfort and consistent fit; the bi-flanges insert the deepest, for maximum isolation when a seal is achieved, but when I move my head the seal shifts ... I avoid the tri-flanges but many users love them. You don't have to play music as loud with the Etys in -- the isolation means you don't have to "drown out" outside noise. The Vibes can really crank, which is good if you like it loud. If you require isolation, as on busses, trains and airplanes, the Etys would serve very well. If you don't mind noise intrusion in noisy environments, the Vibes would serve well. (Sitting in your quiet room, you might enjoy the Vibes more than the Etys; riding a subway, you might enjoy the Etys more.)
The Vibes have a fairly "open" sound for in-ears; the Etys place the music squarely between your ears and are very precise and detailed sounding. One way to describe it is that the Etys are "neutral" while the Vibes are "fun" ... the Vibes pump up the volume and sound fuller than the Etys ... the Vibes are especially good for techno, house, dance, electronica, whatever (but I've found that the Etys handled the massive bass cleanly on those, whereas the bass can be thin-sounding on older, less-loud recordings). The Etys are especially good for classical, acoustic, etc. .... but you can listen to just about anything with either one.
Basically, all this is a long-winded way of saying what exX08 said. I like both. A lot depends on how you expect to use them and what you listen to (both sound good on iPods, for example).