Checking now (say for Monday), the quickest bus time I see is 4:45. I see a train with one connection that takes about 6:00. So not quite two hours longer - and you do get to get off and stretch your feet for almost 20 minutes in the middle of your ride. But you do have to get off to change trains, lug your bags from one platform to another, switch tracks, worry about being late (20 minutes isn't exactly a tight connection - but who knows?).
I personally find long train rides relaxing and fun and dislike long bus rides, so the choice would be easy for me. I like to get up and stretch my legs, even on the train, and I hate feeling stuck in a tiny bus seat. Some people really don't mind a long bus ride or don't see any real difference vs. a train - so they'd rather take the bus to get there faster and without a change. Personal preference!
Checking the train now, the one 8:05-14:02 with a change in Pragersko that goes through Maribor doesn't seem to make that many stops (twelve total?). Not exactly a local train stopping all the time. I took a similar train ride between Maribor and Salzburg a few years ago, and the scenery from the train was lovely. Could be from the bus too - who knows?