I'm surprised no one has mentioned the E320 CDI yet.

E320 CDI Blutec

Base price: $52,325
Fuel tank: 21.1 gal
Weight: 3,860 lb
Emissions: Tier 2 Bin 8 (Bin 5 w/ AdBlue)
Trunk: 15.9 cu ft
0-62: 6.8 seconds
US EPA MPG: 27/37

GS450h
Base price: $55,615
Fuel tank: 17.2 gal
Weight: 4,134 lb
Emissions: Tier 2 Bin 3
Trunk: 7.5 cu ft
0-62: 5.9 seconds
US EPA MPG: 25/28

The Lexus contributes more to climate change and foreign oil while the Merc contributes more to air pollution (still a lot cleaner than our Bin 10 '03 Odyssey). I'd go for the more practical (actual trunk and longer range) Merc myself, especially since it can run on biodiesel. Actually I'd go for the upcoming 530d (with BMW's version of Blutec and AdBlue), which is even more efficient according to the Euro cycle.