aluminum won't break unless you ride it for 20 years or it's faulty or you have a wreck that would kill any other material just as hard. don't buy a cheap frame(get something that is well made) and don't wreck and it's just as durable as steel.
so ask yourself...
are you going to ride this frame more than 20 years?
are you going to repair this frame if it gets in an accident?(bent tubes, dings don't count)