Overnight train is by far the best as there is pretty much nothing to see if you do it during the day. It will be an experience if nothing else. Take some card/board games and some beer/vodka and time goes by :)
Border crossing could be painfully slow though.
Both cities have more luxury (watches, cars, restaurants) going on than you will see in the rest of your lifetime. Some really good places but not cheap.
In Kiev there is not that much to see outside the central churches/square etc. Chernobyl is an interesting visit and I suppose very ‘in’ these days.