it mostly depends on what you plan on doing with the bike.
if you're gonna thrash some street or dirt jumps, get a BMX.
if you're gonna ride it on trails and such, and use it mostly to get around town, get a mountain bike. you can get around on a BMX, but it's easier on a mountain bike.
a bmx has 20" wheels, and a mountain bike has 26" wheels. so a mountain bike is taller as well as longer. also, a mountain bike will have gears...and all the parts will be different.
i'm 22, 5'6", and i weigh about 115lbs, and i ride all kinds of bikes (BMX included), i also know a guy who's 30, 6'0", and about 200lbs and he rides a BMX. it's all about preference.
i'd say get a BMX bike. learn some tricks. have some fun before you get too old and start to regret not doing certain things.