The $70 one is probably the 1st generation Kobo. I'd pay a bit more and get the Kobo 2 - teh Wifi version. I own both versions, and the new model has a much faster processor, making navigating the device much zippier. It really drags on the Kobo 1.
As for the reading experience, the screen is fantastic. Clear and crisp. Sony made a big error with the 600. The screen is inferior to all the competitors out there. (That said, the new 350/650 models have great screens.)
The thing most lacking in the Kobo is a 'Go to page number' function. But we're all hoping we will get this in the next firmware update.
The Sony and the Kobo can all read epubs, so any epub bookstore and public library is a source for reading material. As for pdf books, I would avoid them on the Kobo. I think the Sony can handle them better. But they are still frustrating. Stick to epub, in my view.