The 8900 is a smartphone and the Memoir is not. That said the camera on the Memoir is one of the best on the US market and you can quickly download to all the top photo sharing sites like snapfish and such. If you live in a TMobile 3G area (very few right now) that is available with the Memoir and not the 8900 but the 8900 has WiFi and the Memoir does not.
I went with the 8900. I can't believe I went this long without a BlackBerry, what a device. I am trying to convince my wife that the Memoir would be perfect for her and take one extra item out of her purse . . . so far she is still happy with her old Nokia.