From the stations there are also escalators up to the city center on the hilltop.
Yes, it is really a matter of time (the bus is about 15 min faster since the train has to take a longer route via the Elsa Valley near Empoli which is flatter) and the fact that the bus drops you on top of the hill in the city center while the train station is at the bottom of the hill.
If you want to see what the new “Blues” train serving that line looks, do a Google Image search by entering TRENITALIA TRENO BLUES SIENA. Since part of the line is not electrified this new train has a triple hybrid engine fueled by the electric overhead cables, electric batteries, and diesel. Trains serving this line can also be the “POP” model and the “ROCK” model. You can Google all of them (Someone in charge of naming trains at Trenitalia must like music).
To see what the buses look like enter AUTOBUS SIENA FIRENZE AUTOLINEE TOSCANE, then click on Images. If you want to see the interiors add the word INTERNO (interior) and most images will show the interior.
There is no doubt that the trains (no matter which type) are more spacious than the buses. Just be aware that large suitcases go to the belly of the bus, while on the trains you have to carry them on board yourself and place them on the overhead compartment or in the larger areas at the entrances of each car.
Regarding the crowdness of the trains and buses it really depends on the time of travel. Both trains and buses are heavily used by local commuters.