Out of the two characters, who's the driver and who's the gunner. To clarify more, who's up on the tower providing sniper cover and who's on the ground pushing the button?

Do you get to choose or are they alternating?

Please no spoilers about certain battles, events, boss fights or stuff like that where you do split up.

I'd say Sheva is more of the "sniper."  Even her little figurine that you can purchase has her holding a sniper rifle,  However, there are also times where Sheva has to do the up close stuff. 

Thanks.

That will help decide which character to choose. I'm fine with either but my friend is not that good at videogames.