the two devices are essentially the same thing, they use the exact same SoC (processor etc)
the only difference as you have pointed out is the screen, although the HD2 looks better.
There are some other differences too
The digitizer is newing on the HD7 meaning you can have many point touch screen, the HD2 uses an older version that supports 2 points ( i think) however because of this the HD2 with WP7 has some issues, it works but isnt as complete or functional.
Because of the screen (largely) the battery lasts longer too
and thats about it.
WP7 on the HD2 is fantastic, and works without issue, NODO is perfect and they are working on mango as well, once mango goes RTM its quite likely the HD2 will have it too.
the HD2 supports just about any OS you can think off
The HD7 isnt currently unlocked, allthough im sure thats just a matter of time.
So to sum it up, the HD2 is the most usable phone out there, WP7 on it is almost the same bar some issues (SD card compatibility, camera issues, touch screen isues) it runs android, WM, linux and has almost identical to the HD7
The HD7 has the edge with the touch screen and battery power, doesnt have the issues with some of the hardware that the HD2 has but is either equal or worse than the HD2 in pretty much every other aspect
if it were me i would get an HD2 again, because i can do more with it, until the HD7 is unlocked that is at which point the HD7 would have dominance other than the slightly poorer looking display (all be it with a high bit depth)