I voted for Deities and Demigods the Epic Level Handbook.
I actually really like both books. However, my problem with these two books is that, despite conventional wisdom that would have suggested that the material in DDG would have been a natural progression from ELH, the two texts are mechanically incompatible.
Yes, there are elements in both books that parallel each other (like the acquisition of skills, bab and save progression, etc.), but overall, there are too many glitches between the two similar, but different, rules sets to make them seemlessly work together.
While I understand WotCs decision to only require one to use the three core rulesbooks, this is one place that ignoring this noble (but misguided) attitude would have paid off.