It's been my experience that the urethane orings do last the longest. I'm actually running a mix of urethane and HBNR right now. I have urethane for the orings responsible for cycling (rear can, bolt sail and inside of back latch) and HBNR for the other dynamic orings (front and middle can and bolt guide).
Formerly of DBS Kidz 2, DBSPaintball.com and DBS Titans
2010: 2nd Place D3 Triple Crown, 1st Place D2 Brawl for It All
2011: 1st Place D3 WCPPL E1, 1st Place D3 NPPL HB, 1st Place D3 NPPL Chicago, 2nd Place D1 Brawl for It All
2012: DBS FLOOD - VPL D2: 2nd Place D2 VPL E1, E4 and E5, 1st Place D2 VPL E2 and E3
2013: 1st Place D1 VPL E1, 3rd Place D1 VPL E2