I am using the FIzik Rondine on the tri-bike. Used it for my training and in Ironman Arizona.
My bike originally came w/the Fizik Arione, which was too narrow.

I really like the Rondine; have plenty of thigh room but supports the 'sit bones'. It is also cheaper than the Arione (about $75). I got it from trisports.com.

good luck,
Laura