halacha - Grace before maariv motzae Shabas?

I generally eat s'uda sh'lishis between mincha and maariv, and have been reciting birkas hamazon at night, after maariv. One who does so (and says havdala in the amida of maariv) does not recite the Shabas addition, "R'tze", in birkas hamazon. My question is whether there's any reason to say birkas hamazon specifically before maariv (and thus say "R'tze").

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_{This is relevant to me and thus I asked my local rabbi. He said he thinks I should say birkas hamazon before maariv. This, even though he knew that doing so would take a few minutes from my pre-maariv Torah study with a study partner. However, he did not explain his reasoning.}