I've had both the Tyros 4 and still own the S970. I gigged with both over the past few years.

The Tyros has a great sound and controls for live play. The S970 has a good sound and a much lighter footprint to carry around.

I'm currently gigging with a Korg PA 4X. Has the controls(+) and sound of the Tyros but also has the size and close to the weight of my S970.

If Yamaha ever produces a Tyros similar to my Korg PA 4X, I might switch to that.