I've heard the Altmann and the Benchmark but separately, and it's been a long time.
I didn't care for the presentation of the benchmark... Just didn't do it for me, I don't remember exactly why... however, that was at the time when I learned I didn't like the oversampling sound... and converted to non-os DAC's. I'm currently using the latest Scott Nixon DAC which is really nice. Check the other thread in this area for more info on non-os dac's if you're interested in those.
I can't imagine not being happy with the sound of the Altmann. I personally don't care for its implementation (no case, no battery, etc.) given the price point but I think the sound is worth every penny.
-C