At this point, I think it is mostly a matter of which company you want to do business with - Barnes & Noble or
Amazon. Both companies now have comparable products, with a few slight exceptions. In other words, one might have a feature that appeals to you that the other doesn't have (for example, being able to go into a B&N and ask a person for help is a feature some people like that makes them choose a Nook, while others like the text to speech option that allows some books to be read out loud to you, so they choose a Kindle, etc.)
Personally, I chose Amazon. I've had Kindle products since June of 2008 - in fact, my new Kindle Fire is registered as DVCLiz's 11th Kindle!!! I've been so pleased with their product line and their amazing customer service.
If I were choosing a Kindle for a 14 year old boy and the budget allowed, I'd go with the new Kindle Fire. It's a tablet as well as an ereader and he can play games, surf the web, download movies, etc. It's an all purpose machine.
If you just want a "traditional" ereader, I'd choose the new Kindle Touch. Kids like touchscreen products and the new Touch is a very cool piece of technology. I'd get the wifi only adfree version for a 14 year old boy. The Touch, a nice cover and an Amazon gift card would make a wonderful gift.