if presented with both options, is it better to train at a magic circle or silver circle firm? for long term future opportunities?
I really don’t think you can make this distinction between the SC and MC firms broadly.
Your expectations, aspirations and motivations may make one MC firm the best long term opportunity for you, but then the second best could easily be a SC. For instance, there will be some people whose “best” two could be Slaughters and Macfarlanes, but then the rest of the MC and SC firms are not so great.
I’d try to avoid categorising firms like MC, SC, US and considering all firms in that category as similar as they are often quite different from one another and may have more similarities with firms outside of their category.