Should I get a 2x8GB RAM kit or a single 16GB stick?

Get the 2x8GB kit now. It gives you dual-channel speed right away, boosting performance in games, video editing, and multitasking. A single 16GB stick runs slower in single-channel mode, and the “future upgrade” argument is weak because RAM prices drop and matching sticks get harder to find. Unless you’ll add the exact same 16GB stick within months, take the performance now.