Know this is off topic but I'm debating should I get a 12.9 inch iPad when the new one comes out or a 4k tv? Random question I know. Torn which one has more usefulness but I have a MacBook pro and iPhone already.
It is an odd question only because a 4K TV is more of a fun discretionary buy whereas most people (I think) buy a 12.9 iPad because they are in a graphics or photography related business and it's a work tool.
I have a MBP, iPhone, iPad minis, iPad Pro 9.7. That is where I stopped because the 12.9 serves no need for me. Maybe you have a use for a 12.9 -- like drawing is your hobby, I don't know. Which is my point. We'd have to know why you are considering a 12.9 to evaluate if it would be of real use to you.