Hi. Welcome to KH. You could substitute any yarn that will work to gauge and that would mean a worsted weight should work. Beyond that, I assume it is quite similar to SS which means that if you don't sub with something similar your shawl will have a different drape. That might be OK. Maybe try a swatch with your chosen yarn and see if you like the way it works up in your pattern. HTH
Cheating is an option. . . . Cheaters never win and winners never cheat, but smart knitters who want to retain an iota of sanity do, cheerfully. ~~Kory Stamper
I haven't used the Eco. The Simply Soft is listed as a worsted weight #4. But my experience has been that it's more like a #3 sport weight. It's much thinner than Supersaver. When I've crocheted hats with both, it's been the same hook, same pattern but ends up with a one size smaller item with the Simply Soft.
Thank you so much for your answers, you are both right ,
I tried my yarn and I found it a little too stiff, so will look for a softer yarn.
Am pleased to be here and enjoy talking knit and crochet with you.