For both options the largest costs and environmental impacts are incurred during the construction phase. Once built, both technologies run with relatively little environmental impact. Operational wise, it is unclear which technology is more harmful to the environment.
Constructing enough wind power to meet demand has its affects on local bird populations and impacting the aesthetic quality of scenery. Impacts to bird populations can be reduced by studying migratory patterns and building windmills around them, thus affecting ideal sites to be developed. Also, proposals for wind farm construction are often met with strong resistance. An alternative is to develop offshore wind energy, however this increases construction and transmission costs.
Like wind power, LFTRs does not produce any emissions. Their environmental impact would mostly stem from mining thorium; this amount is very small. For a 1GW city sized reactor, LFTR would only need about 1 ton of fuel per year, extracted from around 200 tons of ore. Since thorium is plentiful and seen as a waste when mining rare earth metals, LFTR would poses little additional environmental impacts, and is arguably more environmentally friendly given thorium is currently a waste product.
2007 World Energy Consumption
A LFTR reactor does produce waste; however the amount can be easily dealt with safely. The reactor would produce virtually none of the dangerous transuranic waste produced by conventional reactors, and which lasts for thousands of years. Instead, transuranics remain in the reactor core and are themselves consumed as fuel. LFTRs also have the ability to create Hydrogen, Ammonia, Methanol, Dimethyl ether, and desalinate sea water from waste heat. These capabilities allow New Zealand to produce its own transportation fuel and agriculture resources. For a 1GW LFTR, 1 tonne of waste would be produced. 83% would loose radioactivity in 10 years, while the remainder would be radioactive for around 300 years. At the end of a LFTRs lifetime decommissioning would incur costs. The reactor core (radioactive from decades of neutron bombardment) would need to be safely allowed to cool for a few decades, and all the construction materials recycled. However windmills like solar plants and power storage stations also have limited life spans and would too need decommissioning.