St. Louis is currently listed as 3rd highest violent crime rate in the country (Detroit is #1, but in years past St. Louis edged out Detroit for murder rate). I'm not saying don't move there, but just be aware that north St. Louis is a pretty bad area and the economy doesn't seem to have improved much, despite the massive investments into the central urban attractions like the Arch, Union Station, and the things listed in the OP.
Perhaps the main problem with St. Louis is the division between the actual city of St. Louis and the various suburban towns that surround it. In 1970, the city boasted a population of nearly one million, and just 3-4 years later it was down to about half that. City residents blame the population implosion on school busing which led to white flight. The burbs like Clayton, Webster Groves, etc., in St. Louis County ("the County" as people call it) had their own tax base and when affluent people flocked there, the city lost a substantial portion of its tax revenue.
Overall, I really liked St. Louis but the crime rates are pretty bad. The weather in the summer is also bad -- very hot and humid. You will need AC, and it doesn't come by default with every dwelling. If it were me, I'd be looking at Phoenix or Tucson, because of their proximity to southern California and of course the weather, but then I spent some time in Arizona and fell in love with the place. Cost of living is probably comparable if not cheaper than St. Louis as well. Can't speak for the start-up community in Phoenix, however; there's not much of one that I heard of, as of 2010 or so.
If you're worried about the violent crime rate, don't move to the south. Texas, Florida, and Louisiana are always near the top for violent crime rate. The states with the lowest murder rates are in the north, generally. You want to move someplace like Vermont or Oregon.
http://www.fbi.gov/about-us/cjis/ucr/crime-in-the-u.s/2012/c...
St. Louis has taken its share of knocks over the years like many Midwestern cities, but those crime statistics are not indicative of the normal experience here.
If that sort of thing doesn't bother you, then you'd probably be better off just moving your startup to Oakland and maintaining a connection to the Bay Area startup network. Or, if it does, move to a more modern, safer city in the South that still has a low cost of living without many of the economic problems of the rust belt cities (and better weather to boot!).
[1] http://www.nbcnews.com/business/most-dangerous-cities-americ...