Dichlor is better for a new fill because it allows you to build up some CYA faster which is good. CYA is kind of like sunscreen for your chlorine. Without enough in the water, your chlorine disappears really fast especially on a hot sunny day. For every 10ppm of chlorine that Dichlor adds, it also adds 9ppm of CYA. Once your CYA is around 50, you'll want to switch to only using bleach so your CYA doesn't get too high.

Trichlor also adds CYA but those tabs dissolve really slowly. You'll be better off just using the dichlor and leaving the trichlor tabs at the store.