It's not a question of one being better - they are both important jobs. I do both so can see the pros and cons of them.
You need the IBMS training portfolio for biomedical scientist registration. There are multiple routes (five that I know of) to clinical scientist registration. But the most common at the moment is the Scientist Training Programme.
That is the three year post graduate training programme. It's very competitive, but so is getting a training role for biomedical science.
What interests you in a career? What is the most important thing in a job for you?
Ultimately if you do a biomed degree at Westminster then you aren't blocking either route in life sciences. A placement year is great but they are very competitive too- you can't guarantee having one.