Nook is kind of falling out of popularity; Barnes will probably sell the company within the next year. That being said, if you'll be sticking to third party ebook sellers anyway, the current Nook Glowlight is cheaper.
Most of the debates are between Kindle and Kobo nowadays. If you're in the $150 range, then it's a fight between the Kindle Paperwhite (2013 model) versus the Kobo Aura. There's no clear cut winner in my eyes. You can look at the hardware specs for each device to see what's important to you.
Also note each book marketplaces. I really like Amazon's for a variety of reasons (good selection, good prices, good cloud features, lots of indies, Kindle Unlimited). Other people may disagree. You can break the DRM on any ebook right now AFAIK and ebook format switching is trivial.
EDIT:
A Kobo Aura sale.