I'm at MMU for library school and it's been fine. It's my experience that quality is really going to vary widely by department- for example, they're not too strong in the traditional sciences or humanities (with the notable exception of a great Creative Writing program) but their Art and Design school is excellent. I think their programs in fashion are highly rated as well, which no doubt comes from the university's history as an ex-polytechnic.
The University of Manchester is an all around very good university- obviously I don't know the specifics of their fashion program, but I don't think you can go wrong with them. They are of course bigger than MMU though so they'll have more resources with regards to library materials, student support, international student advice and careers info. (Although if you end up going to MMU, I can give you my personal experiences with student loans, etc.)
In short, I think go with your gut. If you want to go to uni over here this coming year and you're willing to pay the extra $4K, I'd accept Manchester's offer (universities raise their tuition every year anyway so it's likely that waiting another year for MMU would almost eliminate the price advantage anyway.)