Well the Tabs are different now. You can take up to $360 off the price of your desired new phone and (probably) keep your plan, but the Tab charge will be added on top instead; in this case it's $15/month. However the iPhone 5S won't be $0 down. It's an expensive phone. So the upfront payment is currently $350 with that Tab plan.
There's also the Tab plus, which gives you a bigger discount on the phone, but the plans are more expensive (starting around $80+ after Tab charge).