What are the biggest differences between the PIX OS and ASA OS, if any at all? If I have two ASA's in production and need to test config changes on a PIX in a test environment, will this be sufficient?
The latest ASA software is 8.0(4) and that was released August 2008. There is an equivalent PIX release so if you download the software from Cisco and upgrade the boxes the PIX will serve well as a good test bed.
I'd suggest getting version 8.0 (4) on both your ASA and PIX. If you don't want to upgrade your ASA devices, then I would suggest running the PIX on exactly the same version as you ASA so that your testing environment is a close reflection of your production environment.