i prefer maximum comfort when i need to focus 100% which is not often. if i'm doing typical routine tasks like email or lights tasks i can do it on a concrete block and not notice. when i'm studying for the mcat etc., then i need maximum comfort for maximum focus.
i prefer to put my ipad on a bookholder and sit on a recliner which has a high back to cradle my head. since i'm reclined, my back rests on the sofa. it's not perfect but it's the best solution for me.
I've tried ergonomic chairs as well. currently i'm going to look into the steelcase leap & the humanscale freedom chair with headrests. the herman miller embody chair is no good for me because it has poor lumbar support in the upright position. in the reclining position, there is no headrest to support your head. HM says that the chair stabilizes the cervical spine (neck) by supporting the thoracic spine. that doesn't work for me. when you recline, your neck has to counter recline, and without a headrest this is not comfortable.
i also recommend doing neck and back exercises to strengthen your muscles so the fatigue less.