The Tyros models have more features, such as chorus voices, and three upper voices instead of two. It also has the ensemble function, that is like a very advanced harmody function that let you play instruments together, and the keyboard makes it sound really natural. The 975 is a light version, but still a very good keyboard. I have an S975 myself, and also have a used Tyros 5.
If you decide to go for the top model, I would consider the Genos. T5 is now six years old. With the Genos you get a lighter keyboard than the Tyros, a touch screen, and many new features. And if you are satisfied with the 975 there is no reason to get a bigger and more expensive keyboard. Only you know what is best for you