I am also a Sheff student and also biased. I will no doubt be staying in Sheffield after my degree because I love it so much.
One of my best friends goes to Loughborough and while she loves her degree, she visits all the time and wishes that she could pick up her degree and bring it to Sheff. Loughborough is a very small town and while the campus is awesome, it can feel a little claustrophobic and cramped at times. Sheffield is still a relatively small city, enough that you feel safe and you can get around relatively easily, but big enough that there is soo much to do and definitely big enough that you can see yourself staying here for work after your degree.
I do molecular biology which I have found a very difficult science so I understand that for you as well the course is going to be the biggest factor. I'd say go where you think the course is going to be most enjoyable. Don't bother looking at league tables because 1. things change year to year, and 2. all big unis (Russell group or not) are accredited and the courses will be of a very high quality, ultimately with minute differences between them overall. Look at modules and the core values of the course (Research? Concepts?) and the flexibility of modules, the overall basis of the education (how broad is your first year? Broad is great for building a career) and THEN look at the city. But ultimately, you will be there for a long time. Again im biased. But pick sheff... !! I am currenly on placement year and I am ITCHING to get back to Sheff and my beautiful peak district !! ❤️