Well there are upgrades in the 2013 model, but if you do not need them, you could just get by with the 2012 model. The software is able to be updated, whenever the updates get pushed to that specific device. The 2013 Nexus 7 may be supported by Google slightly longer than the 2012 model, but I don't think it will be too different.
You just need to decide between the hardware difference:
- Speed
- Rear Camera
- Texture and material on the back
- Size
- Screen resolution
Heck, depending on what you do on your tablet, you may not even notice any differences between 4.1 and 4.3, as 4.3 was primarily under the hood updates. One of the more noticeable changes from 4.1 to 4.2 were lockscreen widgets. So it depends on what you need and want from it...