Tipping Point has fewer abilities for packet capture (called logging in Cisco-speak) while Cisco can capture a defined number of packets past the evnt, Tipping Point only can capture the packet that caused the event. This means that if you plan on providing analysis of your security events, you will not be able to tell if an attack detected on a Tipping Point IPS was sucessful. Tipping point does a much better job of looking at asymetrical traffic (one side of the session). Cisco has the option, but it doesn't work very well and last summer actually increased the CPU on a 4270.
I'd say get a Tipping Point if you want to set it and forget it, and get a Cisco if you are really investigating your events.