My best advice on the subject would be that it depends on your mounting situation and the tool you intend to use. How much leverage is going to be applied to it? On our other tables we oversize the slide and cassette to provide extra rigidity, because when a tool is mounted on it, especially with routing, and it is offset from the mounting surface a large amount, it can create a small amount of movement at the cassette which translates into a lot of movement at the tool.
Obviously running dual slides with one cassette a piece will improve a certain directions rigidity, and running two cassettes on the same slide will help it with another. For Plasma cutting the system doesn't need to be nearly as rigid as a routing table because there is no force being applied against the torch body.