The S4. Yeah I know they are very similar as far as their function, but I know Tbol can help a little with mass. If I put on 5 quality pounds and ripped out a little more I would be really happy, so I'm not looking for much. Mainly wondering, if it would be a waste of one or the other to stack them, cost not being an issue. Also wondering if I would gain any extra strength from stacking them.