PagerDuty also includes voice calls, which we've seen through experience are more reliably delivered than SMS messages (esp. SMSes through email-to-SMS gateways). As with the SMS messages, you can immediately acknowledge or escalate during the phone call using touch-tone.

I also think PagerDuty's ability to graphically define the on-call schedule and escalation rules is much nicer than mucking around with Nagios's configuration files, but I'm a bit biased :)