I think that both could work, depending on how you go about it. For open shirt binding, here is an excellent tutorial on how to do it comfortably and safely. Please note the chest sizes that are capable of doing this without risk of injury and proceed accordingly.
For the prosthetic, anything with Latex is able to be adhered to skin. The problem lies in the fit of it. If it is loose in any way, it WILL come off as the day progresses, usually sooner rather than later. Spirit glue, which is the usual method of latex prosthetic assembly, can hurt and tear the skin even when applied over a small surface if removal is done incorrectly. You MUST have proper removal procedure and instruments/chemicals if you're going to be using spirit glue all across your chest, side, and part of your back.
The advantage of the prosthetic is that you can do detailed realistic scarring that can be adjusted and fixed with ease, and will stay on as long as you don't pick it off. The disadvantage only lies in the potential for the base piece to come off as you stretch, move, and sweat throughout the day, and the damage it can do to your skin, but only if you do not remove it with care.
The advantage of open chest binding is a hell of a lot less hassle with trying to make something stays on (I've never had an open chest binding wardrobe malfunction), breathability for your skin, and being able to adjust the scarring without potential damage to a base piece of latex. The disadvantages are non-permanency with the scarring, potential harm if you do the binding wrong, and possible wardrobe malfunctions. Essentially you just need to decide which you want to try first. If one doesn't work or is too time consuming, try the other!