Mainly just comes down to the type of game you want to build, many factors. The main thing with side scroller and top down camera FOV is you lock the players movement to certain axis, to allow the game design to control certain mechanics and progress through the game. Do you want multiple leveled buildings, elevators, jumping across cliffs, etc… this would be mainly for a side scroller. Top down view is for games like RTS, old school dungeon crawler RPG, space shooters, etc…