The answer is going to depend tremendously on the fuel used, the conditions of burn, and any stack scrubbing or recapture equipment present.

Internal combustion engines have a starting advantage in that they tend to burn "cleaner" fuels - especially in the past. But external combustion systems have the advantage of larger plants and (generally) more space and attention to efficiency and emissions.

Looking at coal alone, there's a lot of variation among coal fields, as well as categorical differences between soft brown coal and anthracite.

But in any case, a cleaner starting fuel gives less to mitigate at the stack.