Neither. We don't recommend generally that you put pucks in the skimmer. When the pump is off, the dissolved chlorine can damage components. Better to spend $10-$15 on a floater and use that.

Usually, we recommend bleach--there's never ANY reason not to use bleach as your chlorinator.

However, if your CYA/Stabilizer is low, and your pH is high (like in new concrete/plaster pools) , Tri-Chlor pucks are a great way to fight the high pH, add CYA, and keep a constant level of chlorine in your water. But when CYA hits 50ppm (or whatever your desired maximum is) you need to switch to bleach.

Also if pH takes a nose-dive and dips below 7.3, or below 7.0, you should stop using the pucks.