It all depends. I liked the V600 better than the V400 for these reasons.
It was made out of Metal/Aluminium instead of plastic.
I seemed to be able to hear better on it. Although i think it was do mainly to ergonomics.
I liked the look and feel of it much better. To me the V400 was the beta, and the V600 was the final product.
I value style, and durability, so I went with the V600. Also, I had a V60 which I loved. Possibly the best phone I have ever owned, and the V600 remind me much of it. I'm sure was a big part of my decision.
And as for battery life, if I remember correctly it was about the same. No huge difference.
So I guess if your an average user, The V400 is a good fit.
BTW, I dont think Cingular sells the V600 anymore. I think they replaced it with a 500 series phone. Which is made out of plastic 🙁