Your servos aren't rated for 2s Lipo voltage. You would need a regulator for them, which adds weight and is another failure point in the system. The 2 cell LiFe/A123 batteries are the perfect solution, IMO. They are the lightest choice, can handle more current than your servos can ever draw, are durable, and hold their charge forever. I'll suggest going ahead and getting the round cell A123 instead of the flat cell LiFe batteries because their improved durability will hold up better to gasser vibration. The only real drawback to LiFe batteries is that you have to balance charge them which may mean buying a new charger and figuring out how to access the balance plug. I made a charge connector for mine that lets me charge through a standard 3 prong servo connector that I run to a charge port in the fuselage. It works great.