There is a difference in both sound and configuration. On the latter point, the Proton is readily portable since the DAC runs off battery and is recharged via the USB. The Benchmark is configured in different ways but in one version serves as a preamp with multipe selectable inputs.
But you're asking about the sound.
I've listened to both at different times and places. I had already purchased the Proton before I had the chance to hear the Benchmark. But what I read about the two units in advance seemed to be validated after I had the chance to audition. Note, however that I already owned the Proton -- so I can't say if that swayed my impression. I think not -- as it would be easy to sell the Proton if I had really greatly preferred the Benchmark. To summarize, both are very good units and it will come down to your preferences as to which works best for you.
I would say with the Benchmark you have the experience of being able to hear very deep within the recording. You really hear everything and identify very small sonic differences with this unit. So, I think you would say this has great resolution. There's not an ounce of muddiness anywhere. It's quiet and is nicely designed.
With the Proton, I immediately liked the portable design and the way that Wavelength set up the physical volume control to follow the computer volume. The sound is attractive and full. I felt that the tonal quality of the music was immediate and present -- not at all distant. So this is quite an engaging unit. I listen to it for hours on end.
The sound of the Proton is clear but I don't think you get the same feeling of scientifically precise resolution that you get with the Benchmark. On the other hand -- I hate to say it this way -- with the Proton I feel like I'm getting more music out of it. Maybe it is that the midrange is particularly good -- or that the emphasis on carfully resolved highs is less. I'm not enough analytically minded (analytically eared?) to be able to give it to you in those terms.
You'll know already that the brands use a different approach to the USB interface. That also played a part in my decision -- but I'm not sure it should have. Also note that the Proton takes a good while to break in fully. But as I'm an AKG K-702 user, anything less than a century feels like short break in to me.
