It's an even trade-off, really. Giga Drain makes it take hits better, but it can be argued that Sceptile is not catered to take hits in a Specs set. Energy Ball lacks the HP gain, but it has a 10% chance to lower the foe's Special Defense; and it can be argued that it's too low of a chance to rely on. They both net around the same KOs, both OHKOes Deoxys-D. I just prefer Energy Ball, since the 10% Special Defense drop can make a difference in certain situations.