Thanks so much for the feedback. Exactly what I was hoping for. When I use the laptop on location, I just want my Art Director to see images pop up as I am shooting. Any heavy lifting is done back in the studio on an Intel Mac Pro, Eizo monitor, etc.
I may do a bit of video transfer, and I know Compressor will be much faster on the 15.4, but that is not my primary need. And as you said, with travel being as challenging as it is now, smaller is better.
I'm using the prior generation 2.53Ghz 13" with 4 GB of RAM to shoot tethered with the 5DII and find DPP and Capture 1 work great.
A friend has the new 15" and we tested shooting tethered side by side. At capture there's no noticeable difference. Opening CS4 or working in C1 there was a difference, but not what I expected. Batch processing is where the 15" would kick the 13"s butt. But I so rarely do that on location it's not an issue.
We also calibrated the two side by side, under the same light in the studio, and found there was more color gamut on the 15", but the difference was subtle.
The 13" is a lot nicer on the plane. It fits comfortably even when the big fat guy in front puts his seat all the way back.