Here is a very good “compare and contrast” article between the two contenders for real-time web protocol champ. Neither one is particularly easy to use yet, but that will shortly change. If anyone knows of a client reader for either one that is available as a Firefox extension: let me know. A Twitterific-style desktop app would also be good.
Overall, I believe that both PubSubHubbub and rssCloud represent a huge step forward for the web. While I personally believe that PuSH is a better choice, competition is always good and will make both protocols stronger.