I started with an HTC Vive and upgraded to an Valve Index. For me the difference was night and day.
The HTC Vive ran at 90hz/FPS and had a resolution of 1080x1200 per eye. I used super sampling in the Steam VR settings to bump the resolution up so that I could read the cockpit gauges a bit better. This helped some, but there were still usually a bit blurry.
The Valve Index can run at 80, 90, 120 & 144hz/FPS I set mine to the 144hz/FPS setting. The resolution is 1440x1600 per eye. For me, the higher resolution and much faster refresh rate adds a lot to the experience. The movement/motion feels so much better at 144hz/FPS than at 90hz/FPS.
I'm still super sampling from the Steam VR settings as it aliases the image and removes the jaggy edges from things.
The cockpit gauges are crisp and clear and easy to read at a quick glance.
Another thing to note about the Valve Index is that the sound quality is phenomenal compared to that of the HTC Vive.