Hopefully not too late to chip in.....
I've had the original Kindle for years - mine was sent over from the US and originally tied to the US store, one of first ones with a white case. I travel a lot with work at it's been great buy - never needing to worry about the battery or ever running out of books abroad. Sitting with the family on a beach in Greece and being able to order the next book in a series I started was great. (Mine has 3G built in, your wifi cover may very!)
Scroll on until last year when I got a Nexus 7 Tablet and installed the Kindle App. Plus points are all the benefits of a Kindle, with much better rendition of newspapers - with good colour photos. I have a £10/month subscription to the Telegraph that I mainly read when I'm away from home and sometimes at the weekends. I can also watch movies and do all the "tablet" things, including calendar, email, web and some games. Downside is the battery only lasts a day with serious use.
Last Christmas both my Kids wanted tablets, so I bought them both a Kindle Fire HD. Not as technically good as the Nexus, but with better parental controls. They can get at all my books, I control what they buy with a password and I can also lock out the Web and YouTube. The device is great; it does run its own version of the Android operating system and is very much designed as a consumption machine for Amazon Services. The kids love them and they work well.
So, for the Kindle - it's cheaper, the battery lasts forever and you can read it in full sunshine by the pool. You'll struggle to read a tablet in the sun. Also, you won't kick youself as much if you lose or damage this one.
For the tablet (and I would say buy a Nexus 7 if you're an adult, as you can easily load the Kindle App). It's much better for reading the paper and doing more in general. You can read it in bed. You do need to watch the battery.
Me - I still use my old Kindle. It stays by my bed for night time reads and also goes away with me when I'm off to the beach. Normal travelling, I carry the Nexus where the Kindle app is a bonus. On a long flight I can watch my own movie then read the paper and a book.
PS - If you're looking to replace the iPad, check out the Nexus 10 or a Samsung Tab. Both cheaper and better specified.