Which is more serious: multiple sclerosis or lupus?

Hard to answer. Either of these can be a very serious, difficult to treat illness. Some forms of lupus are fairly benign, some cases of MS are quiet without many symptoms. Either can lead to a healthy life with a normal life span though.
Both are significant. But, both are potentially treatable and controllable, and advise that MS be handled by neurology and lupus by rheumatology.