If Cycling is your only exercise, and you are facing issues with your back you could also consider mixing it up a bit.
I mainly ride full susser now, although in London I am hard at both ends. The terrain I generally ride on dictates what style I ride, I use a beater bike to go to the store.
I would advise anyone with serious back issues to complement their cycling with other exercise. Kayaking, whilst not so practical fora ll, is amazingly good for muscular back issues. Swimming, Yoga, Pilates, walking are all great.
When I fist damaged my back when I was 13 I could hardly walk for six months, the things that saved me were cycling and kayaking.
The only time I have had issues since have been after periods of enforced sedentary lifestyle through other injuries.
The sit up and beg posture is certainly more relaxing when getting back on a bike after time away.
But the more your weight is at the rear, even standing, the less you can get air over those holes.
Depending on the design, rear suspension can snap into every depression.
Good riding technique on moderate surfaces can actually mean you feel these things even less on a light hardtail, as long as you are anticipating things, in terms of your back.
I am rarely in the saddle on a hardtail over anything remotely rough.
Another thing with full sussers is that people tend to run their saddles lower than ideal, and this makes it more of a chore moving off and on it.