I don't know much about the English courses- as I applied for law- but I much prefer Sheffield as a place to Cardiff, it has a much nicer feel in my opinion- Sheffield is more my kind of place i guess. Coincidentally, I applied and had offers (both grades the same) and chose Sheffield as my insurance over Cardiff.
If i were in your situation I would definitely put Exeter as my firm, from the perspective of reputation alone as I have no idea about course structure etc. The grades are higher giving you something higher to aim for which can never be a bad thing! Then Sheffield/Cardiff depending which place/course is most suited to you!