Cardiff uni student here!
I'm aware that I'm slightly biased in my advice so I think that it would be helpful for you to definitely do more research, however, the key to Russell Group universities is the research that takes place there. At Cardiff, the people teaching you are active in their fields and this can lead to some great opportunities and internships.
Can I ask what you aim to study? The ranking really does depend on the subject, though I would argue that it's more valuable to check out the modules of what you will be specifically learning on the course, then you can compare this to the other universities you are interested in to find the course that's more relevant and beneficial to your interests.
I hope this helps! Please let me know if you have any questions specific to the university.
All the best
Jaz - Cardiff student rep