Using different chocolate brands will change the flavor of your brownies. That's ok, I found I liked the combination of both dark and semi-sweet chocolate. A "dark" chocolate is one with 60% or more cacao. I used Ghirardelli bittersweet chocolate baking chips but there are other brands you can use. For the semi-sweet chocolate, I used Callebaut chocolate chips (44% cacao). Try to find good quality chocolate, but it doesn't matter if you use chips, chunks, or cut up a bar. You're going to melt the chocolate down anyways.
If you want to frost your brownies to add sprinkles, it's easier to frost the cooled brownies in the pan prior to cutting. I would chill briefly before cutting.