Should I add an 8GB or 16GB stick to my laptop with 8GB soldered?

Add the 8GB stick. It keeps dual-channel mode, giving better performance in everyday tasks for less money. With 8GB soldered, adding another 8GB gives 16GB total running in dual-channel, which helps in games, video editing, and browser tabs. A 16GB stick gives 24GB total, but only the first 8GB runs in dual-channel; the rest is slower single-channel. Unless you need over 16GB for huge datasets or multiple VMs, the 8GB upgrade is cheaper and faster.