I'd keep the kindle, but then I'm biased, I have a kindle and I like the experience Amazon provides in shopping for ebooks and support for other devices such as tablets. I don't have the paperwhite version so I can't comment on how well the light works for reading, but when the light isn't needed and you're sat in bright sunlight on the patio, you can't beat eInk, whether it's a kindle, nook, sony or any other eInk based reader.
As far as which to go for, it doesn't really matter too much, there are ways to convert books bought from competing stores (with or without drm) to work on each device. Having a kindle doesn't mean you have to only buy from Amazon, you can buy from any DRM free store (or with a little more legwork, any store).
Since you've already got a kindle and it was a present, I'd say keep it. It'd likely be my recommendation even if you were still looking to buy anyway
