The Sony models you mention are no longer made. If you find them, they're old new stock which is fine or used or refurbished. I don't personally like buying refurbs but you're buying not me.
I can't stand the Kobo. I think it's a horrible device. I don't like the user interface, it's unstable and overall I just hate it.
If you like Sony's get the T1. The 505 is many years old & not worth considering. You can get a T1 right now for $100 - $120 depending on where you are on the planet. US had them for $100. Cheapest in Canada is $120. I don't know about elsewhere.