Well, GS400 vs GS300 isn't going to help you with snow. Both are RWD and will be bad in the snow.
In some instances, the Lexus V8 is not that much less or even more efficient than the 6cyl models. For instance the LS gets better mileage than my ES, assuming its a 98+ model. The ES is underpowered for its weight.
Look here:
http://www.fueleconomy.gov/!Removed!/findacar.htm
According to the EPA, the 400 gets 1MPG less than the 300.
If you commute 50 miles a day, 5 days a week (250 miles a week or 2000 miles per month) at say, 20MPG vs 21MPG, the GS400 would cost you $500 a month at $5.00 per gallon, the GS300 would cost you $476.25. So $24 a month more in gas to drive the 400.