I have both, and in some ways, I, Claudius is superior because it's Claudius looking on. Claudius the God is Claudius in power, and it can't quite cover Nero since, you know, it's Claudius first person POV. And only in covering Nero properly could you come close to those fascinating chapters about Caligula. There's some nice stuff with Herod, I admit, but on the whole, you just don't get the full effects of the characters plotting since they're plotting against Claudius now. I've re-read I, Claudius but never felt the compulsion to go back to Claudius the God.
Also, thanks to I, Claudius, I had spoilers for Rome that were decidedly squicky and freaked out my roommate, so yay, Robert Graves
What kind of literature and what kind of life is the same question. - Tom Stoppard