Just got the got a GX, but they had to prep it so they gave as an 02 LS430 as a loaner for the day. SO from the little I could compare, as the dealer is 40 miles form our home on a freeway with some bad spots, here is my opinion.
The LS has less wind noise, but the GX seems to have less road noise and more isolated from bad roadsurfaces. On sections of the freeway that were graded the roadnoise made itself very know in the LS while in the GX we hardly noticed it. The GX also seemed to ride a bit softer over uneven pavement. The LS felt more powerful and handled better (firmer suspension), while the GX wth the shicks on normal setting tend to have felt more floaty over uneven freeway sections. Oh and I peeked at the gas mileage indicator, the LS did more than 20MPG with a smaile on average, while the GX was more like 16-17. Ok the GX was brand new and power and gas mileage will improve, but expect a pretty good hit on gas mileage from what I saw. Ok this was only like a one day comparison, but on highway cruising comfort in the area I live I would take the GX due to the difference in road noise and ability to soak up bad pavement better. Here in the Bay Area we have lots of graded and uneven freeway, if you dont the LS might be a smoother option.