When choosing a deck there are two different types of mounting: top mount and drop through. Both have their particular advantages which I will cover:
The original, classic way to mount a longboard. The deck is bolted on top of the trucks, this has its advantages.
First and foremost, top mounting gives the biggest allowable footspace on top of the board. With no drop through trucks coming through the surface, riders with big feet should appreciate a good top mount. Top mounts also provide better road grip at high speeds because at a full lean your weight tends to be more over the center of the board than a drop through.
For this reason, the majority of speedboards on the market are top mount, however there is no lack of drop-thru speedboards as well.
A more recent development in longboard truck mounting is the drop thru. Instead of mounting your board directly on top of the trucks, specially made decks have holes designed into them so that a truck is able to fit through the board. This mounting technique puts the rider on a lower platform.
Sitting on a lower platform provides advantages. Being lower to the ground provides stability but sacrifices grip at high speeds. Drop thru boards also tend to be easier to handle and control while sliding, if that is your idea of fun.
Being lower to the ground also makes drop thrus easier to push because you dont have to bend your knee as far to pump.
Finally, many prefer drop thru boards because they almost always feature wheel cut outs which allow the rider to fit bigger than average wheels. On a top mount the board sits above the wheels, so when choosing a board wheelbite could become an issue.