LiFe battery for sure.
Lipo must use voltage regulator, and the problem using lipo in nitro car is the chance of getting involve with problems I describe below :
1. Cheap made voltage regulator produce low amp, which is not adequate to power high amp servos.
2. Cheap made volatge regulator probably dont last long.
3. Extra wiring and connections.
4. Fuel spills mishap >>> electrical problems.
5. Voltage regulator clutter the car's look, like in the foto above.