The IC+ train takes around 5 hours and I'd say is a better bet time-wise. Paying for 1st class is worth it - you get more space (2+2 seating vs 2+3 seating).
You also have the option of a number of overnight trains, which take 7-9 hours, depending on the train.
Motor Sich has a pretty old fleet (they use an AN-24 on this route) but flies into IEV instead of KBP, saving you a good 40 minutes on transit time from the airport.
UIA has a new fleet but flies into KBP.