I think it mostly depends on which kind of weapon you use.
A sling is a good tool to shoot high caliber or small caliber, in different ways. Locking the sling around your support arm while shooting is pretty good for stability (with the real stuff) and might help your accuracy some with airsoft, but not really that much.
For small arms caliber weapons such as a MP5, a sling is very good if you adapt the europeen shooting style, also called 'instinctive shooting' where you push your weapon away from you, locking it with the sling (3points are good in this situation) making the gun very steady for quick bursts.
Aside from that, slings are rather good for walking back to the respawn...
As for Bipod... i think they are heavy and useless.. If you are heavy into defensive positions throughout milsim day games, a Bipod is good as it keeps your rifle steady and ready to fire when unmanned for a period of time, but that's about it. It won't help you shoot anymore then a locked in arm sling technique.
my 2 cents.