If the hardware is an analog unit, there can be no comparison. It will always sound different that a plugin. If it is digital, though....
In theory, a software plugin and a hardware unit employing the same algortihms would yield the same results. In practice this is very rarely true.
The hardware has convertors and other circuitry that influence the sound in subtle ways.
Also, hardware manufacturers are notoriously secretive about the algorithms they employ in their hardware units, for good reason. Lexicon will never release a software plugin of any of their reverb hardware for the very reason that they fear piracy of their algorithms.
However, what we have been talking about here is the use of a hardware compressor as a limiter to tame peaks before they get to conversion. Once he signal hits conversion, it's too late. Since plugins work after conversion, they cannot be effective as limiters in this application.