I upgraded to the Forerunner 965 from a 945. After using the 965 for a couple of weeks I would say there are two significant improvements, neither of which is a 'must have':

1. The new AMOLED display of the 965 is a lot better. It is very bright and clear compared to the previous MIP displays found on previous generation watches, including the 945. It also has  a much higher resolution (454x454 pixels vs 240x240). On dynamic apps like the compass it really does look great.
2. Touchscreen functionality. You can swipe to navigate through screens and select items, and on maps you can pan around and zoom with your fingers, which makes the limited screen far more useable for navigation. On previous gen watches like the 945 you have to use buttons for everything, which is slow and clumsy at least as far as maps go.

A couple of negatives: I did find the appearance of maps on the new display a bit disappointing.  You'd think they would be a lot more clear with all those extra pixels, but no. The 'dark mode' is horrible, and relief shading just clutters the map. They might work better outside of urban environments, but I disabled both and set the map to less detail, in order to keep it easy to read.

I also found that touch was not enabled on maps out of the box, which is a very strange choice by Garmin. You definitely want to enable that to access pan and zoom.

So: Is it worth the upgrade? If you use the mapping applications a lot then I would say yes. Being able to pan with your finger is a game changer. Otherwise, the functionality is very similar to the 945. There are a few more apps but nothing that you are likely to miss.

Both the 945 and 955 are available at a steep discount now, probably because a new model is on the way. The 945 (and presumably, 955) are still very serviceable and will do everything you need in the gym or out in the wilds.