We are talking about the difference between leaving a seat open for a year, and not leaving a seat open at all. We are talking about the difference between the Senate conformation in normal sessions, and trying to get a Senate conformation literally in the middle of an election.
There was no reason to not give Obama's nominee a vote, there was more than enough time to do so. They even had the conformation hearings in their spare time.
There is no way to get enough time to have conformation hearings for trump's nominee. With Senators running for reelection, you would have to call them all back for weeks. They would literally be working on election day. I doubt the Republicans will even support that.