I pondering whether I should upgrade my Android system, or stick to my current one. Is there any reference website that would list Android devices and see how different Android versions compare in terms of performances (which means some benchmark should be defined)?
I have seen some benchmark websites that compare different Android devices, but my goal is to compare different Android versions for a given device.
It depends from which version you are going to upgrade. For example, if you are going to update to 4.4.x then yes, you should do it. Version 4.4.x is a big leap on any Device.
See Kitkat 4.4.x release notes.
Other versions don't always have a positive impact on performance, for example, the performance of Samsung's version of 4.3 was worse than their version of 4.2.2.
Hope it Helps.