Depending on the kind of diving you do, a rebreather setup can be much kinder on the body than the equivalent open circuit set. For example, one advantage of my rebreather (the Dive Rite Optima) is that, for shallower diving, you can use the unit with the supplied onboard bailout (no offboard bailout tank required). This brings the weight of the unit to 49 pounds. That's still more than a standard 80 setup but very close.
The advantage of this kind of set up is that you can go on long dives well beyond the length of dive that a standard 80 cubic foot tank would allow. For example, you could go on a 60 foot dive for over three hours without incurring mandatory decompression. This also can mean not having to climb onto a boat more than once from the water, which can be a whole lot kinder on the back and knees. There is also no need to change over tanks between dives, which can also be hard on the back when wrestling with gear in a rocking boat. You also don't have to carry two tanks to and from the boat, car and dive shop.
Although I carry significant bailout on deeper and technical dives, I only have to carry the full weight once briefly before hitting the water from the dive boat. Afer the dive, I hand up my bailout tanks to the boat staff before climbing out of the water. This means that I am carrying much less weight for the climb up the ladder than I did when I used to strap on a set of double steel tanks. (Depending on how you configure the unit, this could be 40-50 pounds lighter.) This also means that my knees and back do not have to work nearly so hard when the boat gets back to the marina at low tide, requiring a high climb back onto the dock.
For cave diving, I carry my bailout tanks down to the water before putting on the rebreather. I put the bailout tanks on in the water, and I remove them before exiting the water. Again, I am carrying a significantly lighter load when climbing out of the water (not to mention during the walk to and from my parked truck in the parking lot, some distance away--and usually uphill from--the water's edge) than if I were in doubles.
There is more to carrying dive gear than just during the dive. Carrying rebreather tanks to and from your vehicle, such as when taking to the dive shop for fills, couldn't be any easier compared to a set of heavy doubles: you can even carry just one of the small tanks at a time if you prefer. For those of you that don't have a vehicle with a low entry point, lifting a set the doubles into your trunk is a great way to get a back injury. With rebreather tanks, there is nothing to it.
Really, it is not much of a contest. I sold my double steel 120 tanks right after I got my CCR back in 2006, and I have never looked back. My back and knees are very thankful for my consideration of their long-term health.
