Add the 8GB stick. 16GB is plenty.

You get 16GB total, which is enough for almost everything, and you save the cost difference between an 8GB and a 16GB stick. 24GB would be a nice-to-have only if you’re regularly hitting swap with 16GB — video editing, multiple VMs, that kind of thing. For everyday use (browsing, Office, light coding), 16GB is fine.

Warranty-wise, both options are identical. You’re just filling a free slot either way; neither adding an 8GB nor a 16GB stick voids your warranty. The soldered 8GB stays untouched.

The only reason to buy the 16GB stick is if you know for a fact you need 24GB. Most people don’t. Spend the $20-30 savings on something else.

Future You won’t miss the extra 8GB.