This is something of a "chocolate or vanilla" question, as which of these two you would likely prefer boils down to your system/speakers characteristics and your personal tastes. Nord Acoustics likely offers these two because they contrast each other well, but their sonic differences in this buffer are more of a refining feature than a defining one, and you’d likely be happy either way.
The Sonic Imagery 990 leans to the resolute and transparent while and the Sparkos SS2590 gives a touch of richness and a more "analog" flavor. No one knows your system and preferences better than you, so it’s a judgement call with no right or wrong answer. In this Hypex platform, I would personally lean toward the SS2590 as a safe bet given my personal experience with it. It’s not a huge commitment should you ever want to experiment with tweaking the flavor.