Should I buy a metric-only, SAE-only, or combo fastener assortment?

It depends on what you work on. Across r/Tools and r/3Dprinting threads, the consensus is that 3D printing, electronics, and most modern consumer goods use metric (M2–M6), while older American vehicles, appliances, and US-built furniture lean SAE (#6-32, #8-32, #10-24, 1/4-20). If you do general home DIY, a combo kit is usually the better value; if you only service one ecosystem, a single-standard kit gives you more pieces of the sizes you actually use.