In reality, the answer to this is probably heavily dependent on what they will be doing with Ignition.
IMO, however, a solid foundation in PLCs/Programming cant hurt if their primary function will be to design interfaces with the PLC's. Understanding what can be better handled at the device level is sometimes overlooked but will payoff in the long run.
AB CompactLogix are a good choice if you want physical hardware to test against.
I'll throw a plug in here for @pturmel's free Simulation Aids Module. Useful with or without physical hardware.