Working Memory is how much you can hold in mind at once; Processing Speed is how fast you operate on it. Distinct cognitive resources that combine in different ways.
Assess your cognitive processing speed — how quickly you can take in information, make decisions, and respond. Covers decision making, information processing, mental arithmetic, and pattern matching.
Assess your working memory — the ability to hold and manipulate information in mind. Covers memory span, sequence recall, information processing, and sustained attention.
Taking both gives you two angles on the same person — you. Many users find the discrepancies more interesting than the agreements.