They are about three different varieties of Orville -if you contact support@eventide.com they will tell you the differences.
When running the same preset there will probably not usually be many audible differences – this is the wonder of digital audio. The later models may have improved converters or improvements to the presets, but this will not usually be dramatic, or in itself worth the upgrade cost.
The main reasons to upgrade would be if you want the FireWire, extra AES and more and better presets.
Can't easily give you the dates – I'll guess and say 1999 and 2006.
