Should I buy a metric, SAE, or combined fastener assortment?

It depends on what you work on. Newer electronics, furniture, bicycles, and most imported machinery use metric fasteners, while older domestic automotive and many US construction projects use SAE (inch) sizing. If you don't know what you'll need, a combined metric + SAE kit covers the widest range, though per-size quantities are usually smaller than a dedicated single-standard kit.