NYC's another interesting data point (but merely that) -- violent crime rate is roughly the same as Phoenix's... one city with a heavily armed citizenry and one virtually defenseless, same results. Obviously, other factors matter more.
If you go down the list of US cities with violent crime problems in order, the unmistakable trend is they're all at the bottom end of the economic spectrum. Safe bet that the local unemployment rate has a lot more to do with crime than gun laws.
http://en.wikipedia.org/wiki/United_States_cities_by_crime_r...
I could see Houstion being a little more laid back about recording every little gunshot. For one, guns in texas are sorta like bread and butter. For another, it doesn't look good to have an excess number of reported gunshots in a given annual report.
Houston also has a significant low-income population, and experienced white flight to the suburbs on a massive scale through the 70s and 80s; leaving behind a wrecked shell of a city (downtown has only recently started to come back to life; when I lived in Houston, downtown real estate was dirt cheap, but everyone was afraid to go there after dark).
In short, I don't think it's anywhere near as simple as "guns lead to more/less crime".