Theoretically, in a real life, P1dB is more important than IP3, because IP3 value is an imaginary point (never reached) that indicates when the amplitude of the 3rd order products equals the input signals; when P1dB actually is a real measurement of the compression point which shows when the distortions in the amplifier occur. Usually the amplifiers should be operated below the compression point.
Anyway, (also in a real life) the 10dB difference rule (between P1dB and IP3) could be easily broken (up or down) by the design of the amplifier, especially by the bias filtering design of the amplifier.