Off the top of my memory, Justin, referring to the 9.6 volt lithium ions, said that he would not want to go much higher, i.e. that is pushing the envelope. He also said that the power cut-off circuit cuts in at 6 volts.
Generally anything outputting in the 7 to 9 volt range is going to work fine. From there you are not going to get a definitive answer as to the best choice. As you said, there advantages and disadvantages.