I've owned the Benchmark and currently own the Atma-Sphere Class D. I personally think they sound pretty different.
I found the AHB2 to be extremely clean sounding, very solid bass, and highly resolving. It had a super black background and you could hear into the recording like crazy. I also found it to be a little lean sounding and 2 dimensional, not much front to back soundstage. I liked it but at the time had a Pass X250.8 and preferred the big Pass amp. The AtmaSphere has the super black background and solid bass, but sounds much more like the Pass to me. I don't think it is as resolving as the AHB2 but it has more the color/tone of the Pass with better side to side and front to back imaging compared to the Benchmark. To me, the AHB2 is a great tool to analyze a recording, where the Pass and AtmaSphere bring you closer to the music/soul of the recording. My preamp is the SimAudio 740P (used on all 3 amps). I would go with the AtmaSphere if you have the AtmaSphere preamp and am happy with the AtmaSphere sound signature.
Cheers