Well, it depends on what you're doing. If you're going to be doing a lot of running around with a lot of close quarters engagement, then odds are, you'll be using your sidearm a lot more often. Having a retractable stock for the sniper rifle would be good since it will be on your back more often.
But, if you're going to be sticking to your sniper rifle more often, like sitting and playing overwatch, then having a full stock would be better for a more stable shot.
But, if you're going to be sticking to your sniper rifle more often, like sitting and playing overwatch, then having a full stock would be better for a more stable shot.