The Eris ultimately saves you $100.00 over the course of 2 years. You pay the same price for your data plan as you would on the Droid which has much better hardware. Sense UI is nice, but if it's a must for you then I'd suggest waiting for the HTC Passion. This is HTC's long awaited phone with a Snapdragon processor (similar to the Moto Droid) coupled with Sense UI. Some reports have this phone being released for the holiday season, but it's a tough one to call.
The Droid Eris is built on old hardware that can get sluggish at times. Initial reports say that Verizon's version is faster than the Sprint HTC Hero but the videos I saw don't show huge performance improvements. I honestly think you should fork over an extra $100 for either the Droid or hold out for the Passon which will probably come in at $199 with a contract as well (this isn't confirmed anywhere, just my assumption).
If you have the phone for two yours buy into hardware that is fresh and powerful, it's an extra $100 over two years. If you are having a tough time justifying $100 today then Verizon's plans may be to much in the long run anyway. I could be wrong on this point, but I'd hate to see you throw that much money on a phone that costs the same month-to-month as a much better spec'd phone.
Good luck in your choice, either way they both are really nice phones. I may just be pickier than others, but that's my 0.02.