Level 2 charging is slightly more efficient in terms of electricity consumption per charge. A few studies have show them to be from 3-12% more efficient depending on conditions.
On one of the Volt forums, owners reported consuming approximately 1 less kW per full charge using a L2 charger. That might only total a $1/wk, which isn’t much, but it does add up. A L2 charger could also prove to be convenient should your driving habits change over time. There may also be local or power company rebates available to offset some of the costs to install a L2 charger.
I don’t ever expect to recover the cost of purchasing a new vehicle. Installing a L2 charger could help reduce the operating cost of the car while at the same time adding some convenience.