The AF ON button is a button for my thumb on both Nikon and Canon that allows me the control of when I want Auto Focus to actualy be on. I turn off the Auto Focus on my shutter and only activate Auto focus when I puch this. This is great because I can focus on an object (not in the center of the frame) and then recompse and fire without my camera trying to re-focus. This just gives me a little more control and makes me a split second faster to get the shot.
The second button is amazing...and sadly not on Canon cameras. This is one of the custom buttons on the front of the camera (the Fn Button) and conveniently right where my finger rests against the camera body so I do not need to search for it. I have asigned this button to switch my metering method from evaluative to spot metering. This is great because I love to use both metering systems while manualy metering a scene. I know some great phoographers that know how to use the Zone System with their spot meter so fluently they do not need to cheat (like me) by using the matrix metering...I only wish I was there. I generally meter with the matrix system and then spot meter in complex light situations to make sure it is right.
This just works for me.