What style notched trowel should I use to lay 6"x24" ceramic on a shower wall?
Most tile manufacturers will have a specific trowel recommendation for their tile. It may not be either of the ones you own. In that case you'll buy another one. In any case, it is normally clearly stated in the installation instructions for the tile. The size and shape of the notches affects both the total amount of thinset (so a 3/8 X 3/8 X 3/8 notched trowel lays down an amount of thinset that would be 3/16" thick if flattened completely) and the way that the ridged thinset interacts with the tile as it is placed (V's have a bit more give than square-notches as the tile is embedded.)

Read and follow the tile manufacturer's installation instructions.