Coming from 6 weeks of the Hero (with Sense UI, essentially an Eris for all respects) and 2 weeks of the Samsung Moment running a stock 1.5 android, I can't say there is *that* much of a difference from a Hero w/Sense UI & 1.5 to Android 2.0 on the Droid.
In response to your question about battery life, there are some threads on how to maximize battery life, and I can tell you from experience with the Hero, disabling WiFi when it is not needed, and getting rid of the HTC clock/weather widget that seems to constantly try to get a GPS "fix" will definitely improve battery life. (just add the HTC Widget for WiFi on/off to your home screen)
Only having had the Droid for 3 days now, I haven't found all the subtle differences, but I can say that the HTC Sense UI definitely has a few things going for it, and a few draw-backs as I see it.
Pro Eris w/Sense UI:
1. Much nicer dialer, being able to search contacts by typing a few letters in a contact on the keypad - I know, there are alternative dialers in the Market that will run on the Droid and Moment. Installing one of these is a *huge* improvement over the stock dialer, HTC's is just included and works very well.
2. 7 home screens is nice, especially if you are going to run some of nice HTC widgets included in Sense
3. Bringing us to some useful widgets included by HTC (and *some* CPU/battery wasting, poorly designed widgets.)
4. I love the HTC calendar widget, look, feel, function over the stock calendar app. (Again, alternates are available)
5. Lighter, smaller, way more convenient to turn the phone on with the physical button by your thumb, rather than having to reach up to to corner where the power/wake button is on the Droid. I'm not sure I'll ever get completely used to the location of the button on the Droid, and I'll still prefer it on the Eris/Hero. It' just a bad spot where Motorola decided to place the button, and it's "slightly under" the trim, plus you use it so often, it's just a big inconvenience that you will experience tens of thousands of times during your relation with the Droid.
6. I personally prefer the charger location at the bottom of the phone, however I do realize the Droid was placed where it is to facilitate the car and home docking units.
7. Mini USB port on the Hero just seems sturdier than the micro USB port on the Droid, plus I have half a dozen Mini's lying around, which of course, is just a personal thing, but it might also apply to someone else. I also have a few car chargers with Mini, but not micro USB ends on them. (I'm assuming the same on the Eris - I haven't been able to get the Eris out of my daughter's hands for more than a few seconds yet, and Haven't really looked at that yet :smileywink
8. The roundness just feels a bit nicer in the hand.
9. "Pinch zoom" in the browser if that really matters to you. Apparently to Apple fans, this is some critical piece of motion which is missing from most Android apps, possibly some sort of motion akin to bowing before their deity or something. I don't know. I used it on occasion, but more often I'd just double-tap the screen and the font would be perfect for my aging eyes. I guess it's a personal preference.
Cons for Eris/Pros for Droid:
1. Android 2.0 does give you Google Navigation, and there are a growing number of apps in the Market that are simply not available for 1.5
2. Faster processor coupled with not having the "overhead" of the Sense UI definitely makes the Droid more responsive. I noticed this with the Samsung Moment also. Where the Hero would sometimes get a bit (or a LOT) laggy from time to time, especially things like waking up, alarm not responsive to touching the "snooze" button for 10 seconds, often not answering the phone by sliding the finger down the first time (or first 5 times). I just got in the habit of always pressing the physical answer button on the Hero. I have yet to experience a single lag on the Droid.
3. By *not* having the Sense UI running on top of Android, it seems like updates to newer versions of Android should be much easier (and faster) to port to the Droid than it will be to the Eris/Hero or any other "highly customized" interface. I know it's possible HTC is delaying releasing a 2.x version of Android for the Eris at the request of Verizon for marketing purposes, but Sprint doesn't really have a good reason to delay the upgrade to Android 2.0 on the Hero, which would put it on a better footing competing with Verizon, and Sprint will not commit to anything other than 1st half of 2010. I suspect the reason is not that it hasn't been requested, but that HTC is either having problems porting the "proper" CMDA stack in Android 2.0 to the hardware, the Sense UI to 2.x, or they are just taking their own sweet time and not dedicating enough resources to the project, or a combination of all 3 factors. I think we'll see the same with the Moto Cliq, Sony custome interface Androids, and Samsung units with their touchwiz interfaces... Although my understanding with Samsung in the past is that they aren't particularly reliable wen it comes to software updates on any of their products.
4. The Droid has a "flash" for the camera which could be a big plus for some.
5. Physical keyboard for those who use it. I have certain things I like a physical keyboard for - Passwords, SSH client into Linux systems etc. I tend to use the virtual keyboard about 90% of the time.
6. Because the Droid is getting all the attention, I think you'll see a lot more work in the area of custom ROM implementations for the Droid, although xda-developers are not slouches either, and only develop for HTC products. (Of course this is only a factor if rooting/heavily customizing is your thing.) Along the same lines, we might find, in time, that there are more cases/accessories for the Droid, but at the same time manufacturers of stuff for the Eris also will be co-developing for the Hero, and the combined numbers should make either market attractive.
Really, choose which ever suits your personal preferences. Both are great devices