I see you've also posted this same question on audiosciencereview. Switching power supplies, properly implemented, are technically superior to linear power supplies. Looking at the specs for the Nord, it uses Hypex power supplies, which are known to be of very high quality. I don't know why ATI chose linear power supplies; perhaps they felt their target market would not accept a lightweight amp. Perhaps linear power supplies were cheaper for ATI. My advice: pick one and move on. Personally, I prefer switch-mode power supplies, but I don't care for Class D amps, so I'd probably choose the Benchmark Media amp, which is a switching power supply with a Class AB amplifier, unfortunately more expensive, and only available in stereo or bridged.
If cost is an issue for you, I'd choose the Nord. I don't think the ATI's warranty is worth the added cost, but that's just my opinion.