Ripple supporters will say that XRP is superior because it doesn't require mining, but this is a seriously contentious issue - let's remember that the Bitcoin mining process is not arbitrary. It introduces a specific type of security based on proof of work. While Ripple removes the proof of work concept, it may not require burning CPU cycles, but it is thus not protected by any proof of work system. Maybe proof of work isn't necessary, but Ripple will need to prove that over time, whereas Bitcoin's system has proven amazingly resilient over four years in the wild, with every hacker in the universe trying to exploit it.
Because there's no reliable way to find consensus by IPs or anything network related, Bitcoin's method of finding consensus is proof of work: once CPU (cycle) one vote.
The way I understand it, the important difference and reason why Ripple does not need proof of work is because it has something reminiscent of actual accounts. JoelKatz compares Ripple's method of finding consensus with how people in a room would come to consensus. That's only possible because you know what people are, what a person actually is (as opposed to within the Bitcoin system).
So the security in Ripple is rooted conceptually in its social aspect, not in the code. It's actually not free (you need XRPs) and it takes effort (setting up trust relationships and/or gateways) to create an account, which is something that cannot be so easily automated. Kind of like a clever CAPTCHA.