More or less, the 315 has full buttons, and is black or near black. Its style mimics the 505. The 315 also has S-video out digital/optical audio, and 2-rows of A/V RCA connectors. It has the security upgrade slot on the back. It was the last full feature legacy line receiver.
The 305 has only a power button, in a browny grey case (first ones anyway), and may or may not have just one row of A/V. Some are known to have S-video and optical audio.
It was a replacement for the more basic 301, which was big with a lot of extra space.