As noted, it really is nothing more than a matter of choice. Yes, the rumor is that the Eris will be brought up to 2.0 (or 2.1)... but just because it gets the same "rev" does not entirely mean that it is "caught up" to the Droid.
The screen will never be the same size or resolution. There is no GPU in the Eris. And the MSM7201 - 528 Mhz processor is a far cry short of the OMAP processor in the Droid. So,when all is said and done, you will still have a lesser phone.
That being said, there is still nothing wrong with the Eris. It is (I believe) the best looking of all the "Hero" phones, and whether it is the squarish European edition, the odd looking edition on Sprint or the Eris, the phone is well liked.
Really, what it boils down to is what you want to spend. If you are willing to pay up for a better quality phone, get the Droid, if price is a major factor, then by all means get the Eris, it may not be up to the level of the Droid, but it is far better than even some more expensive WinMo and Blackberry phones.