Good questions.
Many people like a 7" tablet for traveling or keeping in a coat pocket and a larger tablet (~10") for home.
Dalingrin, one of the developers responsible for CyanogenMod (a variation of Android called GingerBread) on the NC, is currently porting it to the HP Touchpad in preparation for the next version of Android, Ice Cream Sandwich (ICS), due in a few months. He has stated that the Touchpad will get ICS, and "maybe" the Nook Color. I'm not sure if this is a time problem or if the Nook is too slow for ICS. Time will tell.
Nook Color:
New: $250, refurbs or used versions available.
Single core cpu. Stock: .8 gHz, capable of 1.3 gHz
7" screen, 1024x600.
Wi-fi only, but can be tethered to a cell phone.
No GPS. GPS location service available via wi-fi only.
Marginal BlueTooth with a range of 12" when it works.
Sdcard slot for external storage or updating. Max 32 gb.
512 mb ram
No microphone or camera
Virtually unbrickable due to ability to boot off the sdcard. Also able to dual boot via sdcard.
Gingerbread (CM7) may be the last version available
Keyboard, mouse, video cam, storage (usb stick) mounting via usb connection
HP Touchpad:
$200-300, depending on source and 16/32 gb (Craigslist, Ebay, Amazon).
Dual core cpu. Stock 1.2 gHz, capable of 1.8 gHz
9.7" screen, 1024x768 (odd size)
Wi-fi only, but can be tethered to a cell phone.
No GPS. GPS location service available via wi-fi only.
Working BlueTooth
No sdcard slot, total storage 16 gb or 32 gb
1024 mb ram
Front facing microphone and 1.3 MP camera intended for video conference (Skype).
Ability to dual/triple/?? boot limited by available (16/32 gb) storage.
Gingerbread with a promise of Ice Cream Sandwich.
Keyboard, mouse, storage (usb stick) mounting via usb connection.
Both have good battery life and nice screens and are similarly priced.
I think the Nook is a mature platform, but it is slower. Dalingrin has stated that the Touchpad is bogged down by webOS and it will be quite fast with Android.