I'd say Glasgow is the better school, here are my thoughts.
Sheffield is very commonly known as a clearing school and as you'd expect based off the name - Lots of people go there through clearing due to it being undersubscribed.
While yes, lots of English kids do go through to Glasgow through clearing this isn't because the school is undersubscribed but rather due to the tuition fees for English students being £9,250, rather than the £2000? for Scottish/EU Students. So, as you can expect, English students are valuable for Scottish schools and they want to take in as many as possible. If you look at Scottish/EU vacancies for clearing - you'll see that there are a handful of odd courses available at Glasgow.
I'd say its also worth looking at the league tables. Uni of Glasgow consistently ranks higher as a school - and then when you look at the subjects themselves Glasgow has 33 in the top 10 vs 12 for Sheffield.
In terms of prestige and reputation, I would also give it the medal to Glasgow. But this is subjective based on the subject. In my knowledge, the Business school and the Medicine School are top of the league tables and quite well renowned.
In my opinion, the only thing Sheffield has going for it is its proximity to London for career events. But a lot of banks/big 4 accountancy firms are in Glasgow anyway - so it really depends on what you're studying.
Yea that's just my 2 cents.