Smaller plenum takes less space from reservoir, so in theory you get more shots. But, it's not that simple. Usually with bigger plenum you can tune your regulator pressure down to get same velocity, and bigger plenum will also produce higher maximum fpe, but at certain point in plenum size, the plenum size vs max. fpe starts to get really inefficient in terms of air usage per gained fpe.
So in short: I guess it's safe to say that bigger plenum add's more flexibility to tune your gun.