Thumpalumpacus
Major
Mariupol is a major port city with main highways running along the coast as well as several heading inland.
With pressure on Kherson, an advance into Mariupol would not only cut off a Russian supply center, but deny Russian logistics that are using the coastal highway as a route for supply and troop movements.
It would also effectively cut Russian held territory in two, creating a bad situation for the Russians in the southern (Kherson) area, surrounding them with no way out but through Crimea, which is a massive choke point.
From Mariupol, Ukraine force can push toward Melitopol, putting them in range of Crimea (the Kerch area in particular) as well as a partial encirclement of the Kherson region.
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The main reason I'd favor attacking Melitopol would be that it's further from the Russian border and would tax Russian logistics proportionally. It would also have three potential avenues of exploitation, leaving the Russians to guess which one would be chosen. But yes, a drive on either city would split Russian forces, endanger the holdouts at Kherson, and as a bonus, using the Danish-donated Harpoon Bloc IIs, either objective would be a big hamper on Russian shipping in the Sea of Azov.
For those reason, either axis would be a good move if the Ukrainians can deliver it with enough forces to make it stick against the inevitable counterattacks. The difficulty with both is that with the Russians in strength around Zaphorizhye, the Ukrainians will either have to break through the Russians there directly, or offer a flank in the case of an attack on Mariupol. So attacking and securing Zaphorizhye has got to be an important preliminary.