I don't like either. I use a sling.
In the Vintage Sniper Matches, the CMP allows you to use sandbags or sling, but not both. I tried both and found I'm much more accurate with the sling.
It may help using the sling if you can rest the forearm of the support hand on a sandbag or pack, but if the sling is used correctly, that may not be the case.
When properly used, with a good natural point of aim, after recoil you're going to fall back on target with your NPA. Meaning you don't have to "wiggle" to get back on target. This helps a lot in follow through. That's why often in high power you see better groups in rapid fire then slow fire (prone where you shoot both). That's because most shooters, knowing they have less time to wiggle around will get a tighter position.
If the sling and position are correct, as you breath, the sights move up and down, but when you're at the bottom of the breathing cycle (where you should be shooting) you'll find the sights fall back to your natural point of aim.
Bipods nor rest/sand bags (excluding machine rest) allow for this, you'll find you have to muscle the rifle a bit to get back to you NPA, which screws up follow through.
Go to the range and watch people shoot. You'll see both rest/sand bags and bipods, watch them after the shot, you'll see a lot of moving around. Now go to a HP match. Watch the slung shooters. Very little movement after the recover from recoil.
Look at these pictures of the prone position. If you have your NPA, you can't help but fall back into position during recoil, giving you a faster follow up shot and better follow though.