The Taylor reagents have much better compensation against things that might interfere with the test. For example, the PH test can be thrown off by high FC levels. Store brand reagents start having problems when FC is somewhere between 3 and 8 (depending on the manufacturer). The Taylor PH test is good with FC up to 10, and somewhat usable with FC up to 15.
The testing blocks are not interchangeable. The colors are the same, but the amount of water they hold varies. Each brand of reagent is designed for specific amounts of water in the water sample being tested.