IPhone 6 plus or iPhones 7 plus which one is worth buying in your opinon?

Clearly they are both large phones iPhone 6 plus is older while iPhone 7 plus is newer with improved camera. And Water resistance.

But which is better overall and worth buying?

Go for the phone that will be supported the longest and that you can afford.
And if money is not a factor always hey the newer version, unless there's known issue with that particular version.

If I had to buy someone an iPhone I would go SIM-free and believe the iPhone 6 is available if Unlocked but 6s, SE,& 7 can be purchase SIM-free.
I hear the 7 is a nice phone but I might want the standard earbud jack that the 6s' would have.
I might choose more memory or AppleCare accident plan over the plus model unless I planned to use the kindle app very often. Or buy shirts with bigger pockets.