The Leopold is closer to the very centre of the city, the Ibis is a little bit further out (but still walkable). If you're thinking of catching the tram, the Ibis City Centre is right next to the tramline - it's about a two minute walk from the front door to Ponds Forge/Castle Square tram stop. Trams will take you directly to the arena from here.
The Leopold is about a five minute walk from Cathedral tram stop, where you will be again be able to take yellow route and Rotherham tram trains directly to/from the arena. Leopold hotel is also a one minute walk from City Hall tram stop, too, but only the yellow route trams go to/from the arena from here.
If you were thinking of walking around the city centre at night, then I'd probably opt for the Leopold, (but I haven't stayed at either hotel.) I don't know Rotherham at all.