E2 is much more moddable. It also has the JUIX OS, which unfortunately moto hasn't rolled out to much of thier newer models yet. It's also one of the few quadband phones with A2DP and its one of the only non-PDA type phones to have a full SD slot which will get you up to 4GB of storage (perhaps more?) It's probably one of the best feature-packed moto out there, basically what the ROKR E1 should have been.
Unfortunately it's not a very good looking phone and the camera is marginal at best. Throw the E2 functionality and features into a RAZR, SLVR, PEBL or KRZR form factor and perhaps you may have a phone that's close to perfect. Perhaps the MAXX will be Moto's next saving grace.
The W810 has a simpler UI and has much better camera functionality. It also has a Pro Duo Slot that can also accomodate up to 4GB cards, which at the moment is better than the newer M2 format which only goes up to 1GB for newer SE phones.
Both phones are geared towards music but you can't mod or tweak the W810i phone nearly as much as the E2 but the W810 is designed so well you probably won't need to bother. I'd say if you want a decent phone right out of the box with a really good camera then the W810i is your phone. If you like to dig in, tweak and modify the hell out of your phone then the E2 would be up your alley.