As Ab S says the image quality of the Express and Pro versions are the same. What has changed over the years are a number of user controls.
Frankly, many of the changes have not made a large difference for someone like me who is not a 'power' user. However there have been some things which I have found to be very useful. One example is the HDR Tool. In Express there is the option to adjust Highlight and Shadow whereas the Pro version has adjustment for White and Black as well. Particularly with Shadow and Black this allows additional subtlety of adjustment. Another example - which I love - is Speed Edit which is easier and faster than placing the cursor on a slider. The Magic Brush is is good too.

However, these things come at a cost and I think if I was in your shoes I would stick with Express at the moment until you are really happy with C1. If you were going to spend more money I would suggest spending it on DxO PhotoLab 5 Elite if only because of DeepPrime noise reduction and perhaps Lens Correction. It will almost certainly become available with a good discount within the next few weeks, but is still likely to be pricey. If you did buy PhotoLab you might indeed find that you prefer it to C1, as a number of users on this forum obviously do.
This link will give you the history of tools and features added to Capture One:-
https://support.captureone.com/hc/en-us/articles/360012248397-The-history-of-tools-and-features-added-to-Capture-One