Yes there is a difference. The Eris has a 3.2 inch screen, no physical keyboard, and the screen resolution isn't as high. The Eris also is made from plastic, which means it lighter. ANNNNND The Eris is running the 1.5 OS. (Motorola is running the new 2.0).
Now, don't get me wrong, the Eris is an excellent phone and will do all you ask of it.
It comes down to this, YOU!
Go into a VZW store and play with both phones. Set aside an hour or so to do this. You need to see which phone meets what you like in a phone.
One more thing, the first app you download needs to be an anti-virus. There are serveral that are free and do a nice job. Since ANYONE can write and app and post it to the Android Market, it's well advised to protect yourself from malicious software just like you do with your computer.
There are many many differences. What has been mentioned obviously and the fact that the Eris uses a 528Mhz Arm11 Processor where the Droid uses a 600Mhz (underclocked to 550Mhz) Cortex A8 processor. So even though there is only a 22Mhz difference in pure speed the productive speed is higher because of the more efficient processor on the droid. Not a huge deal really but a little better. Also the Droid has a separate processor for video which is nice. The Eris has 288 Mb Ram where the Droid only has 256 which to me helps make up some difference in processor speed since the Eris can cache more.
I would say that if you can use the on screen keypad on the Eris (I couldn't) then you won't be dissapointed in it. Until the Droid came out it would have been the most advanced Droid phone on the market being a slightly more advanced version of the HTC Hero which until the Droid and Droid Eris came out was considered the most advanced Android phone on the market. in the US that is. Europe and Asia have some more advanced ones but what is new really.