I've used both types and prefer the belt stop, provided it is integrated with the production belt.
The problems with the reject is that they can get a little excited sometimes if the density of the product varies, and should be programmed to a "3 strikes and you're out" system where 3 consecutive rejects stop the belt anyway. This is to stop production if an internal component is breaking up.You also need to check the reject bin regularly as it will stop the belt when full.
The challenge with the belt stop is that if you're packing at the end of the line, your packers may clear the line for you, thus removing the contaminated container. Threats are usually sufficient to stop this behaviour though!
All depends on products etc. as always.