How does using hydrogen as the fuel in an electric power plant compare with using hydrogen as the fuel in a f?

You would need to provide much more details as to what you are considering a 'Power Plant'. Can't imagine a large generating facility that is providing power to a community to be using pure H2 as a fuel. Doesn't make economical sense.

Anyway

H2 used as a fuel in a power plant must be more pure than the H2 used in a fuel cell. - Maybe, maybe not. Depends on the design of the particular facility. Probably not.

Burning H2 in a power plant produces many more pollutants than using H2 in a fuel cell. - again, depends on the specific design of the 'power plant' and/or fuel cell. Most likely the fuel cell would be cleaner. But then again, perhaps the 'power plant' is using fuel cell technology.

A fuel cell using hydrogen operates at a higher overall efficiency than would a power plant burning hydrogen as a fuel. Maybe, maybe not. I'm sure I could build a fuel cell that was less efficient than any current power plant in operation.

Using H2 in a fuel cell generates far more heat than burning it in a power plant. - Depends on the design of the two.