Make sure your 100% sure on what university you want to go to before you make your choices for firm and insurance... perhaps read through this page:
http://www.ucas.ac.uk/students/after...ngtoyouroffers
as its got some useful points on it. From what i gather on the page, you have to go to your Firm University if you meet the conditions of the offer. If you don't meet the offer but meet the insurance's... you have to go there. So if you only really want to go to one university... such as KK. then just make a firm and don't bother putting an insurance. I've made oxford my firm (AAA) and wawrick my insurance (AABa and 2 in step I or II) which is one grade less just incase i mess up physics (likely :P). The Wawrick offer could technically be seen as harder because of the difficulty of the step papers though.
Choose Sheffield Hallam anyways, i've visited several times to the University of Sheffield (the other uni in Sheffield) and it seems a really nice city, that is very modernised which you seem to like. :P Have you visited edinburugh yet? It will be a long journey. Decisions down to you at the end of the day. Good luck with wherever you go :)
I couldn't wait to leave school for college.. now I can't wait to leave college for university too.. it only seems like yesterday since i left school though haha.