I understand the urge to lump these gadgets together, but a dedicated eReader and a tablet really can't be compared. If you're husband reads a lot of books, the eReader will definitely get used.
As far as which one is best, I've handled both the Nook and Kindle and they're both great products. I had a personal preference for the Kindle so I purchased that last year. I have the Kindle3 keyboard version--I prefer that to the touch screen (keeps the screen clean).
One advantage of the Nook is that it supports the ePub format--so you can purchase your eBooks from pretty much any online vendor. With the Kindle you're almost exclusively tied to purchasing your eBooks from Amazon. Of course, they've got content covered so this doesn't bother a lot of people, but if you don't want to be tied to a particular retailer that's something to keep in mind.
You can check out eBooks from your public library with both the Nook and Kindle--although its a slightly more streamlined process with the Kindle (can do it wirelessly).
Hope that helps.