There are so many variables in a fire that makes it hard to determine exactly which is better. Photoelectric will almost always respond much quicker to a conventional fire, but an ionization could catch a clean burning gas fire early before it gets out of hand.
Dual Technology detectors will always come out on top. They provide the best of both worlds in terms of the scope of protection. They are expensive (believe me, installed 8 of them to replace the old detectors in my house) but could ultimately be a life saver.
Fine details aside, in the end, any smoke detector is better than nothing.