The DAC 200 can be set as a pure DAC as its volume control can be set to a line level output. I have demoed both and prefer the DAC 200, but take that as you will as I am a T+A dealer.
The DAC 200 has a wider bandwidth resulting in a wider, deeper soundstage. It is also a bit more organic and revealing in character. The 501 becomes more worth it if you take advantage of its room correction capabilities, but sometimes it as a bit more of a sterile sonic signature, obviously sucking some of the color that your room adds to the final result. I have typically shied away from DSP room correction because of this.