Spiral or joined rounds — which one should I use?

In a spiral you never join and never turn. Round 2 starts in the first stitch of round 1 and keeps going, so the fabric is a continuous helix. Nothing marks the round change, which is exactly why stuffed shapes look seamless: there is no line running up the side of the ball.

The cost is counting. You need a marker in the first stitch of the round, moved every round, because there is no visual cue. You also get a jog when you change colour mid-spiral — the new colour starts one stitch higher than the old one ended.

In joined rounds you finish each round with a slip stitch into the first stitch, then chain the turning height and start the next round. The round boundary is a real, visible line running up the work. For a flat disc, that is useful: you can lay it down, count the rounds from the outside, and see immediately if one is short.

The cost is the join itself. The slip stitch and the turning chain both take up space, which leaves a slightly thicker ridge and, if you are careless, a visible staircase where each round starts one stitch to the right of the last.

If the piece will be stuffed, wet-blocked into a round shape, or photographed from an angle where a seam would show, spiral it. If the piece is flat and will be joined to other flat pieces, or if you are working stripes of two or three colours, join the rounds so the boundaries land where you expect.

A practical hybrid that works well: spiral the body of an amigurumi and use joined rounds for the flat base disc, then switch. The base's seam is hidden under the piece anyway.