ecosystems are like wives (or husbands ) ...
If you are going to stick to one for "the rest of your life" , pick the best one! So you won't regret later.
Well, a little extreme to compare, but you guys get the idea.
For that reason, I would stick to Amazon and Kindle. But I must admit B&N had made huge progress since Nook Tablet inception. They are making a more refine reader or tablet with each release. And for a company without the same resources like Amazon, that's a great accomplishment.
I still prefer B&N UI though, but the thing is that B&N ecosystem is still too small, at least for me, even in books selection.
Now, B&N needs to "exploit" those bricks and mortar locations at their max; train their employees, show the tablets to customer, doing demos, that's Amazon's weakness as Amazon is a virtual retailer, specially now that they lost Target and Walmart as physical showrooms.