Your talking about two different things here

SBI is the full ultimate package for $250 whereas squarespace does about half or less for $10 less with the 'advanced' package

With SBI your setting out to earn money and make a business with squarespace its missing a load of key features and it seems to me as if its just a good website builder and statisitics, for one its missing unlimited storage and unlimited bandwidth, it doesn't guide you through how to make a successful website, it doesn't have all the documentation and resources, its missing the tools that SBI has which really help like Brainstorm It and all the others, private lively forums, I'm not going to drag on

At the end of the day if your seeking to earn money off your website SBI is the only tool out there of its kind and squarespace does not compete it looks like glorified web hosting

I'm not saying this just because I own SBI or I want to state your wrong, I'm saying this because if you want to make an online business you would be wasting your money with squarespace