Google Nexus one is in a different league altogether and hence cant be compared at par with the other two; in brief:-
Eris and Hero have same processor (Qualcomm® MSM7600™, 528Mhz), same ram (512MB ROM / 288MB RAM) ; battery is 1300maH on eris in comparison to 1500maH on hero; little design difference like a touch panel on eris instead of dedicated keys in hero; trackball little smaller on eris; eris is slightly lighter due to lower capacity battery; and eris comes with 8GB micro sd card instead of just 2GB on hero (though all these devices support upto 32GB); both have 5MP camera without flash; BT 2.0; wifi; One big advantage of the eris is that it has PROXIMITY SENSOR, which the hero doesnt have, even though it retails at a lower price than hero; but the screen has 262K colour on Hero in comparison to 65K colour screen on eris... gps, compass, accelerometer, light sensor are common; 3.5mm jack and mini usb jacks are also common;
Nexus One on the other hand has a 3.7" screen in comparison to the 3.2 inchers above; resolution is also 800x480 (compared to 320x480 above); But it is on GSM whereas the above are on CDMA; it has 1400maH battery; 5MP camera with LED flash; Double processor speed (Qualcomm QSD 8250 1 GHz); double ram (512MB Flash 512MB RAM); it also has a teflon coated back; proximity+light sensor; 2nd microphone for noise cancellation;
Moto Droid on the other hand is different from this league due to dedicated sliding qwerty keyboard; It has a 3.7-in.; WVGA (480 x 854 pixels); 16:9 widescreen; Arm® Cortex™ A8 processor 550 mHz, 256MB of RAM, and 512MB of ROM, 1400maH battery, it has micro usb jack instead of mini usb; 3.5mm etc. are common with nexus one; It is also the heaviest of the lot; and not only sadikk, other people are also using it on reliance (maybe me also but I am not telling )
Software wise all are running different flavours of Android though within a month or so all should be on the latest version i.e 2.1. And it is Android in fact which is the main point of buying any of these phones; as it is open sourced, highly customisable; with 20000+ applications and growing...though the HTC sense on the HTC devices above (except the moto droid) does make a lot of increase in usability...
There are some niggling issues presently with these devices which we are discussing in the relevant topics; so keep following them for more up to date feedbacks... but they are a class apart from anything we have used so far...
and the most important thing is to START LIVING IN THE PRESENT..... what will come in the FUTURE you never know; if you keep waiting for newer devices, your wait would be endless... so start enjoying NOW rather than later...