I'm just writing down my experiences and thoughts.
In my opinion sitting mediation is a bit harder to perform. when someone does not have the natural muscles on his or her back and torso, the position of the body would be inappropriate, so the chi the air and the blood would stuck whicc is harmful. The same happens when we sitting in a chair, reading or computing. The difference between the two way of sitting is when you're onyour bumjust sitting causally (with loose bended back) for one hour for instance, you're not conectrating on your chi flow.
When you're doing the sitting meditation the chi flow is stronger because youre are circulating it, so the position of your body (=the way of the chi) should be open and clean. Unless the chi flow will stuck. Other thing when you making efforts to keep your back straight, you're going to strain yourself a bit... maybe you point your chin a little downward then the chi is gathering around the neck at the spine and not reaching the foreside of the body, and soon you get a nice headache
But there are many ways do a practice wrong
There is plenty of useful laying practices though which are not too trying. Makes your spine and abdominal muscles stronger and capable of performing the sitting mediations (as well) correctly wthout strain.
Write if you do not agree.
Anyways just like Dvivid said. The brocades should do as well. Have patiance and develop gradually and slowly. No strain, no hurry. Breath calmly and deeply. There is about eight very good word about breathing in the 8 Brocades book I think. I always try to keep those in mind.
I hope this is of some help.