If you plan to use the outer (lateral) AF points when shooting with a wide angle lens, in very dim light, then the wider area coverage of the 600 will make a difference. Also, if you plan to use it with the 70-200 lens, the 600 has a zoom head that goes out to 200mm, well the 580 stops at 105mm. The two flashes actually have the same power, but the longer zoom setting means a slightly higher guide number, thus 600 (60m) instead of 580 (58m).