You can use whichever....the Bars product is really no different than Mendtite. It won't really matter in the long run, both of these items will be temporary fixes at best...the system will need to be flushed when a new pump is installed.
The first item I'd review is where the leak is coming from. Is it coming from the gasket that seals the water pump to the block or from the shaft that holds the pulley? If it's coming from the gasket there's a good chance a $2 gasket would solve the problem. If it's coming from the shaft then cross your fingers....a leak at the shaft indiciates the bearings are worn on the pump. It could break loose at any time rendering the water pump completely useless as well as causing the belt to come off, get wrapped up in the engine, etc.
The quick fix products you're referring to may or may not provide any assistance/extend the amount of time necessary to replace the pump. It's a crap shoot basically...the $7 bottle of mendtite or bars might slow the leak, it might even stop it for awhile...or it might not do a thing. If the bearings are going on the pump it will not be long before the shaft breaks loose...when this happens you'll know it, stop the vehicle immediately to prevent any serious damage from occurring.
Hope this helps!