What state do you live in, is there a deduction for 529 contribution? What is your income?
That would help decide the tax implications. Also, if your income is too high, you can't contribute to an ESA, so that would make it easy to rule out :)
Does ultimate control of the account matter to you? We made a lump-sum 529 contribution around the time of our son's birth. Because of the way 529s are "owned" it still belongs to us even though he is the beneficiary. If shit really hit the fan, we'd have the option of raiding the 529 for any purposes, and paying the tax penalty. (shit would really have to hit the fan for this to happen). From what I understand, an ESA might "belong" more to the beneficiary.
Thinking back though, without a state tax deduction, I am not sure a 529 was that helpful to us.