Side mount (or Side Control, as I like to call it) is great for control because the opponent has very little place to move when you do it right. Also, when escaping side control, its difficult to reverse your opponent so your on top. So side control is a good place to be If you want to be on top, the worst that happens is you end up in the guys guard. There's a whole bunch of hard to reverse submissions from side control as well (kimura, straight arm lock, keylock, arm triangle choke).
With the mount, it's really a high risk high reward situation. If you drill the mount and get good at it, than it can be a dangerous weapon in your arsenal. The problem is that most submissions from the mount either lose you position if the opponent escapes, or you have to dish off to side control. Also, with the mount, the most used way to escape is to buck and get the guy off of you, reversing the position. So if you have a good guard, that makes the mount an even better place, because the subs can flow more freely, since you dont worry about being reversed.