train or bus zagreb to split

Hello, we are a group of 28 students. 

We would like to go from Zagreb to Split, on July 9. Since it is not possible to make seat reservations for trains in Croatia unless at the ticket office in Croatia, I was wondering if we could risk it to book seat reservations only the day before, on July 8, because that is the date we arrive in Croatia. 

We are a big group so I don’t know if that is a good idea?

Or is it a better idea to go by bus from Zagreb to Split and already book the bus tickets now, so we have seats garranteed? 

Thanks in advance!

Paulien