I have only ever bought supermarket ready to roll fondant. I was looking into bulk buying sugar paste on line and it seems to be more expensive. I wondered if the quality is that much greater? I have never had any issues with the supermarket products for my celebration cakes, but would maybe consider the 'professional' sugar paste for a wedding cake if it was worth it?
I guess the answer to your question will depend on the brand of supermarket ready to roll fondant that you use. In Sydney I can buy Orchard brand (I think that's what it's called). I've only used it once or twice when desperate. It's okay - does the job and tastes fine - not vastly different to the brands I can buy at my local cake decorating shop or online. I'm pretty sure that for me, the supermarket product works out to be more expensive that the bulk sugar paste, plus it's only sold in small packets so it's a lot of wasted packaging. And it only comes in white.
It's just the supermarket 'own' brands here. The few I have tried are all the same, and good enough quality for me. I just maybe haven't found the best bulk buy in the UK yet. Any suggestions from anyone would be gratefully received.