I sell AT90S2313 chips in my webshop but also ATTiny2313 chips, what is the difference and are can one replace the other. Well the answer is not that plains simple, yes, both chips are pin compatible and have the same architecture. In fact the ATTiny2313 is an enhanced version of the AT90S2313 chip. If you look at the datasheets you will see that they are almost the same, with some little upgrades in the ATTiny2313. The big question is, are they compatible and can I swap them out. Yes, you could replace the AT90S2313 with a ATTiny2313, this will work most of the time. The other way around will also work in a lot of cases, when the enhanced features of the ATTiny2313 are not used.
So why choose for the AT90S2313 instead of the ATTiny2313, well it is a little cheaper. If you really want to know ALL the small differences, you can download the documentation I found on the Atmel site, it is a few pages of PDF that will explain a lot. Meanwhile I use them both, for the projects that I am making it does not make a difference, only in price of the chip.