Should I add another 8GB stick or a 16GB stick to my OptiPlex 3070 with 8GB already installed?

Add another 8GB stick. It gives you 16GB in dual-channel mode, which makes everyday tasks snappier. A 16GB stick gives you 24GB but only the first 16GB runs dual-channel, and the performance gain is minimal. Plus, an 8GB stick is cheaper and keeps the option for matched sticks later.