While I have to admit that my experience with PSN is limited, my understanding is that XBL allows for easier connections to online friends, and is a better social experience than the PSN.
Again this only from the feedback I have gotten from people who use PSN, my personal experience is primarily with XBL, which I pay for happily if not eagerly. I have no issues with the PSN at all and think it's great that it is free, I just feel that the $60 a year is a decent price to pay for a great online service, though I do wish that there was more free content.
Also, somebody mentioned Netflix, which is awesome.