There are a few other things. The Auto Focus goes to 9 points vs 7 points on the sx. This will give you more focus accuracy. The metering system adds Spot Metering, which will only take a reading where the “Spot” is on helping you control contrast.
I will eventually come down to what you feel you need in the camera. The increased MP is helpful in cropping out unwanted subjects and still having a high MP count left behind. The faster frame rate helps in sports and action shooting.
Hope this helps.

Digital Cameras use a Memory Media instead of film. The ISO settings on the camera are the same as the ISO speeds of film. The benefits of digital over film are:
Instantly see your shots.
Not have to wait until a roll of film is empty to change film speeds. You can change up or down any time with digital.
Not have to learn new settings. The ISO speeds are the same.
MUCH more shots. Your are only limited by the size of the memory. It is not unusual to get over 1000 photos on a 4gd stick.

For the Bird photos you would need a good Telephoto lens. For the Flowers you would need a good Macro lens. In most cases these are two very different type of lenses. Compact cameras achieves this by using digital zoom. That will not give you the best quality photo due to Artifacting and Stair Stepping (Jagged, rough or step-like reproduction of a line or boundary that is originally smooth).
A D-SLR would be best because of interchangeable lenses. This way there wold be no digital zoom to affect the image. Yes...D-SLR cameras are more heavy, but this helps give a more stable image in most cases because you use your muscles differently with the extra weight.
However, both the Bird and Flower Photography would be best done with a tripod. The extreme Zoom for the Bird photos make shaking of the camera an issue and the longer exposure of the Flower photos make vibration blur occur.
Image Stabilization is available in Body and Lenses. Sony Alpha D-SLR cameras have it built into the body so any lens you put on is stabilized. Canon has a line of IS (Image Stabilized)lenses as does Nikon in the VR(Vibration Reduction) Line.

One of the latest high Mega Pixel D-SLR cameras is from Sony. The a900. A full size sensor at 24.6MP.
There is also the Canon EOS 5D Mark II and Canon EOS-1Ds Mark III both at 21.1MP