I don't know full diferences, but one very important is:
Autonumber has a second parameter, called AutoID to create separate list of value for each concept.
For example I have 10Millions of Document ID, then 10 Million of Customer ID also in the autonumber list, and then 200 Product ID.
It's a best practice to separate products on a diferent list of values, because find/insert value product 199 in a list of 198 products is better than finding in a list of 20.000.198 records.
