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We theoretically investigate bimodal photonic crystal fibers (PCFs) for the development of interferometric gas sensors (with one mode acting as the reference arm and another as the sensing arm), working either in the intensity measurement mode or in the spectral regime. For each operation mode we define a merit function that characterizes the sensitivity of the fiber taking into account both the interferometric and the evanescent-field nature of these sensing devices. The evaluation of the above merit functions, which involve the calculation of the propagation constant, the group index, and the field profile of the first two guided modes, allows us to identify highly sensitive microstructured fibers for each sensing mode. In this paper, we propose a single birefringent PCF design that exhibits in both regimes extreme sensitivity in a certain wavelength operation range that can be shifted by changing the air-filling fraction.