If Russia goes to Kherson they're not going to stop until they're through Odessa and into Transwhatever.
The costs to them would be the same either way.
Agreed on the first part of that, but, Kherson could be difficult to secure, long term, making for vulnerable supply lines. Plus, the Russians end up on Romania’s border, which country is a significant NATO member, generally has it’s act together militarily, and is likely to view Russians (especially in this circumstance) on its border very dimly.
Of significance is Mihail Kogalniceanu airbase in Eastern Romania, which is our new air transit center to Afghanistan. It would make Russkis in Odessa (etc.) quite vulnerable, should push come to shove..