The speed difference is not huge (I dont think). If you are happy with the speed of your Eris then thats cool. I dont think there are any major discovered problems... no phone is perfect but nothing huge. Biggest thing might be some people dont like the quality of the camera.
Things to ask yourself:
-Can I live without sense UI?
-Do I need a bigger screen (movies, games etc)
-Would I use a physical keyboard (I heard the HTC virtual keyboard is better then the droids) However, Droid can use swype, shapewriter and better keyboards.
-Does my phone seem slow/do I want,need something faster.
-I have heard the Battery life on the droid is much better
-Do I want a larger device in my pocket.
OK, im tired lol, good luck with the choice, you cant really go wrong. The Droid might have sense ported to it properly in while, ya never know. Choose wisely! lol