IMO, the biggest difference is quality. Pelican has good to very good quality. Surefire has top notch quality. For example, the threads on the Pelican M6/ M6LED are rough, and not as precise as Surefire. Furthermore, the finish is much better on most Surefires.
In terms of beam quality, they are very close. The Pelican M6 LED version IMO exceeds the beam quality of many Surefires. Not one of my Pelican M6 LEDs has ANY artifacts, all have a perfectly smooth beam. My incandescent Pelican M6 does have a few minor artifacts, but far less than my 9 volt lamp Surefires (C3, 9P). Surefire does do a little better job of matching throw to sidespill though (the Pelican M6 LED is geared a little more to throw), while Surefires tend to have nearly perfect balance between throw and spill, plus larger hotspots.
Based on my own lights, here is the BEAM QUALITY categories I would put the Pelican and Surefires. I'm rating by smoothness and artifacts, NOT by type of beam.
Top notch (best of the best):
Surefire L2
Pelican M6 LED
Excellent
Surefire 6P (with P60 lamp), E2E (with MN lamps), E1E (with MN01 lamp)
Very Good:
Pelican M6 (Incandescent)
Below Average (better than Gaglite, Elektrolumens, but worst than the majority of high-end lights):
C3, 9P (with P90 lamps)
To be fair, I could have had a bad batch of P90 lamps in my 9-volt Surefires, but it seems suspitious to me that all 5 lamps I've tried all produce well below average beams in my C3 and 9Ps.