The use of slings can be broken down into four broad categories.
1. Carry aid.
2. Shooting aid.
3. Some combination of 1 & 2
4. 'Cause the Sergeant told you to mount one.
These will blur a bit (particularly with #3).
Carrying from ride to tree stand will be different from hiking a week across the veldt. With the former a svelt narrow sling may siffice. For the latter, a wider band will carry better. But, be aware you may want to vary the carry position when carrying sling for any distance.
As a shooting aid, being able to set the biceps loop (and free it from the stock toe) are middling critical issues. Slings do not lend themselves--generally--to rapid set-up as shooting aids. However, if time is less of an issue, they work quite well. There, the issue is repeatability--set it and forget it.
As to #4, that's much easier. You simply follow the Regulations, keep your eyes in the boat, and not give the Sergeant a reason to make your life "interesting."