When an "A" lens with it set at "A" mode on the aperture ring and the camera set to Auto (Program mode) the camera picks the aperture and shutter speed for you with the only override being the exposure compensation dial. Set at "A" in manual mode changes both aperture and shutter to the correct exposure using the up/down buttons. With the camera set on Auto and the lens not on "A" as you change the aperture on the aperture ring the shutter speed changes automatically to provide correct exposure. In manual mode you pick a shutter speed and use the aperture ring or button to set aperture or shutter speed.
Using a "M" lens with camera in Auto mode, the buttons do nothing. You set the aperture on the lens and camera sets shutter speed automatically. In Manual mode, you set shutter with the button and aperture with a button also or if you want a different aperture you would use the aperture ring and the shutter button to set the correct shutter speed.
Hope I didn't over complicate this...(I think I got lost somewhere myself), but basically both types of lenses will work, just a bit differently. If you want true point and shootability where the camera does the heavy lifting and you just focus then get an "A" lens. I, myself, never use it that way as I like to control both aperture and shutter...as it should be in a perfect world!!
With all that said, the "A" lens used in Auto mode and camera in Manual is a definite time saver and convenience. Hope all this helps a bit.