Rooted (stock rom modified to install/change anything) and CM7 (custom rom not at all like stock) are two different things.
I assume you tried to install CM7 on your NC and now are entertaining the thought of installing it on a sdcard so you can use either rom.
Here's my take on sdcard installs. It's nice to have two different roms, but the sdcard installation is:
. Sdcards eventually fail.
. Sdcards are slower than roms installed internally (aka EMMC), especially when you get the wrong brand and speed. If you want the fastest card, you'd want a 16 gb or 32 gb SanDisk class 2 or class 4 micro sdcard. Use a 32 gb if you intend to copy lots of music, movies, pictures or other large files to it.
Harder to install
. Lots more steps to install on an sdcard
Harder to upgrade
. Most of the guides we have here don't apply to sdcard installs. Follow the wrong guide and you'll have to update your sdcard again.
CM7 will do most things that your stock Nook will, with the except of reading books and running the stock B&N software. This is the B&N reader app that runs on CM7: NOOK for Android by B&N
CM7: tablet that can read books using any reader, but primary purpose is a tablet.
Stock Nook after rooting: reader than can do other things.
Now that I got that out of the way, what do you need your Nook for?