I have both the Nook Tablet (running CM9 off an SD card) and the Nexus 7 (running stock ROM) and I find them functionally comparable & interchangeable for my use (mostly email and web-surfing, some reading, occasionally watching movies). Side-loading content are equally easy on both devices. I sinstalled and use identical sets of apps on both devices (Opera Mobile browser, Mantano ereader, MX video player, etc.).
The NT with CM9 (or CM7) functions just like any full-fledged Android device. Although CM9 has been wonderfully reliable its developers has not advanced it beyond alpha release, so having to update it once a month is a bit of a pain.
I bought the N7 in part because of the prospect of having a Tablet running full Android with stock ROM out of the box. But it does not have an SD card slot, and it turns out that support of USB-attached storage media (to compensate for lack of SD-based storage) requires the N7 to be rooted (and re-rooted following each firmware update)!!!
Hardware wise the N7 is more powerful -- graphic/video is noticeably sharper and rendering is smoother, yet it seems to last longer on a battery charge (purportedly because of Tegra 3's "low-power companion CPU core" feature).
Overall, it's hard for the NT to stay competitive at its original price.