The redbook performance of the Naim and the Marantz are different flavors. The Marantz is smooth and refined with decent retrieval of details, I would call it "pretty" sounding. The Naim has good detail but adds a dollop of boogie on top of the refinement, I would call it "musical" (the CD5 XS is a significant upgrade from the CD5x, more so than the change from SA-15S1 to SA-15S2). For redbook only, I would choose the Naim. I think I would also prefer the Primare which is somewhere between the two in terms of choosing a side of pancakes with syrup or bacon, to use a breakfast metaphor).
For SACD playback - I mean, is there really a choice? The Marantz playing a well recorded SACD will smoke all the other redbook players if the rest of your system is up to revealing the detail, weight and space available in that format. If SACD is important to you - then I would buy the Marantz. You are probably familiar with all the backstory of that format - if you already have a significant collection of SACDs then this becomes a legitimate choice. If you are thinking of getting into the SACD play at what looks like the last act - not sure it pencils out.
For SACD playback - I mean, is there really a choice? The Marantz playing a well recorded SACD will smoke all the other redbook players if the rest of your system is up to revealing the detail, weight and space available in that format. If SACD is important to you - then I would buy the Marantz. You are probably familiar with all the backstory of that format - if you already have a significant collection of SACDs then this becomes a legitimate choice. If you are thinking of getting into the SACD play at what looks like the last act - not sure it pencils out.