How much "cleaner" does summer fuel burn?
Gasoline, diesel fuel, and ethanol-gasoline blends are adjusted seasonally and geographically to ensure proper starting and performance. Winter blends contain higher levels of high vapor pressure compounds like butane to ensure the fuel is sufficiently volatile to ignite in cold temperatures. This in turn makes the gasoline cheaper to produce as it does not have to be as pure as the summer blend$^{[1]}$.

Conversely, summer blends contain lower levels of high vapor pressure liquids to ensure that the fuel doesn't evaporate away in the heat and pollute the atmosphere. This is why summer blends are marginally cleaner than winter blends. A study on winter versus summer emissions near tunnels form the University of Wisconsin collected the following data on non-methane hydrocarbon emissions$^{[2]}$:



As for fuel efficiency, winter-grade gasoline yields $1.7$% less energy on average than the summer. This may seem like a considerable amount, but energy content can vary widely from station to station in either season from $3.4$-$4.8$%$^{[3]}$. 




$^{[1]}$ The Oil Drum
$^{[2]}$Univeristy of Wisconsin, Summer and winter nonmethane hydrocarbon emissions from on-road motor vehicles in the Midwestern United States
$^{[3]}$ EPA, Fuel Economy Impact Analysis of RFG