Worth upgrading tonearm cable?

Which Rega do you recommend? And would I need a new armboard for my SOTA?

Any, the 600 is better but more expensive. There is a new 250 with VTA adjustment for $350. The lack of VTA adjustment is the only problem with the Rega and it can be added as it is here. I cannot say about the arm board; it may fit in the same cutout. Check the length of each arm. If the SOTA uses a wood board a woodworking shop can make a new one and cut it for new arm if you need a new one.

The Rega is a slightly longer tonearm than the Linn, roughly 237-240 mm for the Rega vs. 229 mm for the Linn. If you switch tonearms you'll need a new arm board.

And is the Rega substantially better than the Linn?

I should have specified my reasons for recommending it. The Rega is cast as one piece out of magnesium. SME also does this but no one remotely close to Rega's selling price does. This produces a much more rigid, resonance free arm. Rega can only afford this because of economies of scale as they not only produce arms for themselves but for many other turntable companies. It is safe to say that they are essentially the default choice for those who do not wish to spend over $2000 on an arm. I have used them myself at a time when I was an SME dealer. The SME was better but the Rega was not embarrassed by the comparison.