I've not used a Sigma 100-300 personally but I've heard a lot of folk sing its praises. As for being soft I've no idea but price is not necessarily a guide to softness - the Canon 35-350L used to retail for over £1500 and that was softer than a blamange that had been soaked in softening fluid
Looking at the weight issue, the Sigma is only 1.5Kg, about the same as my 100-400L. It's heavy for sure but it's manageable, I would try it before buying a monopod. I picked up a Manfrotto one shortly after I got the 100-400 for the same reasons your considering one - I've tried it a couple of times in the house to see what it's like and I just don't get on with it, I prefer to just handhold and deal with the tired shoulders later.