Thanks. I went with the IS 600Z. I actually decided on the 400S but when I got there, they had a leftover 600Z from last year, with 0.0 hours on it. They knocked off even more so I got it for $4,750.
For future readers: I loved the feel of the 400S. It felt like a Mario Kart vehicle. Tiny and nimble. It was great. One oddity was that the 400S suspension didn't seem quite as active as the 600Z. Driving them both around the same field, I could see the 600Z front suspension moving the whole time, but the 400S never seemed to compress. Maybe it was a faulty 400S or something odd there, but I definitely felt that the 600Z took the bumps better. Yet, the 400S felt super super nimble and I loved that too.