Is the 6am bus for 475 UAH the one you were talking about with the nicest bus since it's a bit more expensive?
yes, it's one of them.
you can click on the the price and see seat chart. Even if you have doubts - you can ensure which bus model is used by deducing it from chart.
As you can see 6:00, 16:30 and 18:35 - have 28 seat bus (it's not tiny bus, it's the standard size 12 m bus) which is very easy to recognize by 1+2 seat locations instead of 2+2 While on other departures, where price is 315 UAH - there is usual 46-seat bus.
Also you may note, that in there are single seats on left bus side, they are tend to sold-out quicker than seats which form pairs, on right side
Also you can click on (i) Details and see list of sheduled intemediate stops. Convenient bus - have intermediate stops, so they cannot overcome whole distance faster than by timetable, because they anyway are not allowed to depart earlier than scheduled from the intermediate stops, so they can "win" no more than ~20 min, between Uman and Kyiv. While the 6:00, 16:30 and 18:35 - don't have scheduled intermediate stops at all, that allows them arrive to Kyiv in a hour (or even more) before sheduled time of arrival.