A simple dog sling is for brief rear or midsection support. It can be easier to place for a quick potty break, but it does not create front-body support and should not be rewritten as a full-body lifting system.
Rear harnesses add more structure around the back end. They can be a better comparison when the dog needs adjustable handles, leg-girth sizing, bathroom cutouts, or wheelchair compatibility.
The broader support-harness guide remains the better comparison when the handler needs front and rear assistance together. Use ramps, stairs, wagons, floor traction, or veterinary guidance when the problem is not solved by a rear support aid.