I would say that the universities are equally good mathematics departments; some people here will no doubt try and fool you by saying York is miles better. Both York and Loughborough boast medium sized departments, and both departments are actively involved with quality research.
York's undergraduate degree seems academically more traditional to me, and would probably be my preferred choice out of the two for that reason, whereas Loughborough's course seems more practical/applied with the benefit of work experience for a whole year.
The choice is a matter of preference, but I will say placement years can be quite valuable in terms of graduate employability.