For an SVI built up to the same specs as an STI edge you'll pay close to $1000 more. You get an interchangeable breachface, some like it some don't, a schueman barrel, tri-glide trigger,some like it some don't, and you'll be able to say you have an SVI.
SVI will build you want you want from their vast selection of options. STI, you buy a certain model and can add options based on what's available for that model. For an SVI you're looking at a couple of months a least since their is no stock, Fredoom ventures probably has what you are looinkg for ready to go, or can get it from the factory in short order.
They started out the same company, one just went more towards production then the other.
If you plan on shooting USPSA matches STI has the advantage since they offer then contingency program where they pay you $350 or more for winning your class at a match while shooting an STI and wearing a shirt with the STI logo.
I've drooled over some slick looking SVI's but STI is where the money's at for me.