The impression I get is that you prefer Lancaster and want to be convinced that it is as good as Durham. If that is the case I think you should just go there, you would regret going to Durham if you really wanted to be at Lancaster.
It seems like Durham is more targeted by employers which helps with networking etc, which is why the starting salary tends to be higher. If you were applying for some random job I don't think you would get rejected because you 'only' went to Lancaster - the advantage of Durham is that there might be more opportunities to meet representatives from large companies and wangle an interview. The average starting salary from Durham maths is around £25k according to Unistats, whereas Lancaster is £19k, so there clearly is a difference for whatever reason.