Should I buy the iPhone 6s or 6s plus? - 1

Updating to one of them in a few weeks and wondering which one I should get

6s, unless you are ok with the size of the 6s Plus.

Yes

How big are you pockets? Unless you are using the Kindle or ComiXology app a lot on it the 6s should be fine especially if you are getting more memory for the price,
You may also wish to do some math and see what the update actually costs you compared to not updating if your phone is less than four years old depending on why is being offered and if you really getting a discount on the new phone.