IPhone 6s Plus or IPhone 7?

Getting a new phone, which one is better?

The Plus just means is physically larger. The is a plus version of the 7 also.

Better is a matter of opinion. The biggest change with the 7 is that apple removed the traditional headphone jack. So, if you want a headset, it either has to be Bluetooth, thunderbolt (not a whole lot of them) or use the adaptor that comes with the phone. That also means you can only plug one thing in at a time so you can't charge it and use a wired headset.

The 7 is the better phone of course.

The 7 is easily better as it has better features and has newer hardware. Though now the is is the same sooner or later the iPhone 6s will become outdated and will stop being supported while the 7 lives on for many more years as it has more advanced software and you will get used to the slightly smaller screen but if the larger screen is a big deal to you then go for the 6s.