I would say Sheffield is better than Leicester personally, but it's your choice. The firm choice is when you have got all your offers in and you choose the one you really want to go to- that becomes your firm. So then if you get the grades, you go to that university.
Just as a side note, Exeter requires AAA normally, so isn't a place that offers lower grades as someone said, unlesss they have changed their requirements this year.
I applied to 2 good universities, 1 mediocre one and two rubbish ones, in case all goes wrong. My firm was Cambridge, and my insurance Sheffield, so I ended up with two pretty good ones, but thankfully I got an AAB offer from Sheffield for law with Spanish. The other ones I applied to were Queens Belfast (mediocre), and Kent and sussex (not so good) but in the end I decided there was no point in putting down an insurance just because they required low grades (Kent and Sussex both EE) if I wasn't going to be happy there, which is why I risked putting a decent university as my insurance.
Good luck with your applications