Well, when dealing with technology you are always paying for specs/upgrades. Without even doing a comparison in the two I can say that if you are asking that question, you shouldn't be wasting the money on high end smart phones. When I look at phones I usually look at speed, screen (clarity/responsiveness), UI, OS, ease of use, etc. If there is a big enough gap between my current phone to the "new kid on the block" to justify it, I buy the new one.
My girlfriend often asks me why I got my Inc, and there is no difference between it and my moto droid, or iPhone, I just laugh because to her, there is no difference. Much as I can't see the difference in many of her shoes lol. I can say for sure though, that if you can't tell the difference between the Inc and eris, you won't have to upgrade your Inc for quite a while, as nothing will come out that you will see a huge difference in as far as the droid phones.