I would not get a Kindle Original, I have no idea how easy it is to find a K1. I have one but I am not willing to part with it...
It really depends on what you are going to be reading on the device. If you are rading PDFs for work I would go with a tablet of some sort. If you are reading technical books I would still go with a tablet of some sort.
As for reading regular books, the file format really doesn't matter. There are more stores you can shop at for mainstream titles for EPubs but they are all priced the same, thanks to Agency pricing, and the selection is still a little less then what Amazon has under its one roof.
The Kindle allows you to utilize Amazons massive e-book store, largest selection at this time. You can also buy from pretty much any independent bookstore because they sell Mobi and EPub to cover all of the e-readers.