You can tilt under a table. You must tilt at least 35 degreees to get a weight shift to get the preesure off. With a urinary leg back you can get a back flow when you tilt, so be careful. The tilt is a good system because it allows the patients to do weight shift themselves. Slings are made of vinyl.
Firm seat provide better postural control.
But nowadays, you can get a specialty cushion with a firm bottom for sling seat as well. So really doesn t matter. At the widest point of hip and add an inch. If they have braces, measure them with braces on. If you have wheelchair that s too narrow, you ll have difficulty with your transfer.