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Well, let's do some quick-and-dirty math vs. front or side armor. Since any given shot from both weapons is equally likely to have the center hole on the vehicle, let's simplify it out, and assume everything hits.

Battle Cannon: Strength 8, rolls two dice, picks the highest.

27/36 penetrating hits. (4+ on at least one dice.)
5/36 glancing hits.

Plasma Executioner: Strength 7, three shots.

36/36 penetrating hits. (5+ to penetrate times three shots.)
18/36 glancing hits.

Thus, the three shots of the plasma makes up for the extra strength and the re-roll of the battle cannon.

Obviously, this is very quick-and-dirty math: Reliability tilts in the plasma's favor, but "overkill" tilts against plasma. And there's a chance that the battlecannon will scatter behind the Rhino, inflicting a S 4 Ordinance hit on rear-armor. But I'm far too lazy to do all the math / measure a Rhino's rear arc.

EDIT: Beaten to the punch!