Both phones are in the same family. People who notice the most difference between them likely already use a smartphone. I've used palm, nokia, and blackberrys for years. Since you are coming from a non-smartphone, honestly, either phone will suit you well. A few things to keep in mind: you will have to get an data plan, which will run you $30 extra per month. Both phones can open attachments on emails. The droid does have a larger and sharper screen, however the eris also has very good screen (very similar to 90% of other touch screen phones out there).
The main factors you should think about are:
Size: the eris is more "pocket friendly". This doesn't mean the Drois isn't, just that the eris is a bit smaller and lighter, with curved edges.
Memory: the Droid has a 16 gig card, the Eris a 8 gig. This is really dependent on how you plan to use it. With cool apps like pandora and slacker radio that stream music of your taste to the phone, you almost don't need to use the memory card at all. Plus, a 8 gig card will hold about 1600 songs (double for 16 gig). If you decision comes down to the memory card, know that you can buy a 16 gig card for $40 and replace it easily.
Keyboard: I used a blackberry storm (touch screen) before switching. Everyone WILL have their own opinion here. I liked the touch screen keyboard of the Eris better than both the touch screen and the physical keyboard of the Droid. I tested this by trying to type a sentence without stopping and looking at the end to see how many mistakes I made. There were many more on the Droid (using either keyboard) than on the Eris. This was disappointing considering the Droid is supposed to have a better touch keyboard. Which brings me to my last point:
Android version: Android is the opperating system on both phones. The Eris is running 1.5 with a special menu system called SenseUI. It makes the phone look nicer and have more options for customizing. The Droid is running 2.0, which is a newer system but does not have the SenseUI that the Eris has. 2.0 does however add Google Navigation that gives you free turn-by-turn GPS. The Eris has google maps, but not the advanced version that announces when to turn. But the good news is that in the near future, Eris will be upgraded to have 2.0.
Bottom line: both phones are very similar, and to the average user, the decision might just come down to which one looks and feels better to you. The Droid is technically a more advanced device, but the Eris seems to be more polished in the user interface. I went to verizon with the plan to buy a Droid. I was so impressed with the Eris that I went home with one. I did however love the screen on the Droid, it's so crisp and sharp. A dream phone would be an Eris, with the droid screen, running the new 2.0 android with SenseUI. (and rumor has it that the next HTC phone for verzion will launch soon and be exactly that).