Allen Bradley PLC5/20 Processor vs PLC5/30 Processor

Could somebody please explain what are the differences between the abovementioned processors (apart from memory size).
At the moment the processors we use in our plant are all PLC5/20 processors apart from only one PLC5/30 processor. With memory size taken into account, could I download the user program from the PLC5/30 into a PLC5/20 so that we only need to keep stock of a PLC5/20 processor?

There are also I/O limit differences, and communications differences.
Why no go right to the source?
http://www.ab.com/en/epub/catalogs/12762/2181376/2416247/1239760/1551229/tab4.html