Taken from WordPress Feeds:
You can include posts from one of multiple categories or tags in a feed by comma-separating their values. For example:
You can include posts from all of multiple categories or tags in a feed by adding to the end of the link. For example:
I understand the 2nd feed will include posts from both cat1 and cat2. The 1st feed looks, to me at least, as though it will do exactly the same. How do these feed URLs differ?