There should be 2 or 3 threads on this. Given the same ballast and bulb brand and color temp, the SE will give slightly higher PAR. The advantage of the DE is size.
http://www.canreef.com/vbulletin/showthread.php?t=13604
Try it out for yourself.
http://www.reeflightinginfo.arvixe.com/select2lamp.php
Pick the same watt (ex. 250W), the same manufacturer (ex. Ushio or XM), the same ballast (example PFO HQI 250W). Then on one side, pick Single Ended (Ex. 10K SE), and on the other side, pick Double Ended with the same color temp (Ex 10K DE). Pick either shield or non shielded. Click submit.
Example output:
250W Icecap Ballast
10K Ushio SE: 85 PPFD
10K Ushio DE Shielded: 65 PPFD
10K Ushio DE Not Shielded: 78 PPFD
250W PFO HQI Ballast
10K XM SE: 182 PPFD
10K XM DE Shielded: 109 PPFD
10K XM DE Not Shielded: 136 PPFD
250W PFO HQI Ballast
10K AB SE: 130 PPFD
10K AB DE Shielded: 104 PPFD
10K AB DE Not Shielded: 128 PPFD