I have a JBL, and I have never used a nook. But the advantages of the nook are:
- Has e-ink, meaning less battery consumption
- Uses Android OS, which might mean apps in the future

- Has the color touch LCD on the bottom, which is nice. Using the buttons on the JBL is somewhat cumbersome.
- It has WiFi.
On the other hand, the JBL has:
- Uses standard AA batteries. If you have an extra set of batteries, you never have to have downtime as you can just pop in the batteries when you lose your charge.
- More formats are supported. The features supported can be quite different for the book formats (for example, PDF supports Table of Contents, while the EPUBs reader ignores the table of contents).
- Dictionary is nice
- Support for international text