Rotary or oscillating hook: what is the difference?

A rotary hook turns continuously in one direction. An oscillating hook swings backwards and forwards, reversing on every stitch.

What follows from that. Anything that reverses direction has to stop, and stopping and restarting a mass many times a second creates vibration and noise, and puts a ceiling on speed. A rotary hook never stops, so it runs smoother, quieter and faster, and it tends to cope better with thick work and with thread that is behaving badly.

An oscillating hook — the traditional shuttle arrangement, still used on plenty of machines — is mechanically simpler, cheaper to make and easy to service. Machines using it have been sewing perfectly well for a century, and a good one is a better machine than a poor rotary one. It is not a mark of a cheap machine on its own.

Roughly, though not reliably, horizontal drop-in bobbins tend to sit in rotary hooks and vertical front-loading cases in oscillating shuttles. There are exceptions in both directions, so check the manual rather than the bobbin position.

What you can actually observe. Run the machine on a doubled scrap and listen. A rotary machine sounds like a continuous hum; an oscillating one has a distinct beat to it, one thump per stitch, and more vibration through the table.

How much should it weigh in a buying decision? Less than feed quality, presser foot lift and how the stitch looks on your fabric. It is a useful thing to understand and a poor thing to shop by.