I would agree with Nepophyte and Eugene since taking the train is a lot better idea than taking a plane. At least trains run more or less on time and don't get cancelled due to technical problems or weather. Fastflyer makes a very good point re: 2 bed sleeper compartments, they are much newer and are considered "premium" class. So, if you want to arrive well rested in Odessa from Kiev then train would be the way to go.

Also, if you know someone local who can buy the ticket for you then you'll save even more since tickets for locals are a lot cheaper than to foreigners and this applies to train and plane tickets.