For a self defense gun, I'd go with the SD9 + the Apex trigger/spring tax.

If its just gonna be a range toy or teaching handgun fundamentals, nothing wrong with the 909. From what I understand the 900 series has some issues because of it being the cheapo version of the 5900 series. The SD9 is pretty much a straight Glock clone except with no adjustable backstrap (I use skateboard tape on mine). It has a terrible stock trigger like Glocks do though. It's also a double stack vs the 909's single stack.