I have compared the three at a local dealer, and bought the XS. It is quite a bit more detailed than the Nait 5i, with a lot more soundstage information and inner detail which the 5i misses. The 5i does have the characteristic Naim prat, and feels more powerful than its rated output, but less powerful than the XS.
I think that for a careful listener who likes the typical audiophile values such as transparency, detail, soundstaging, etc, combined with musicality, the XS is well worth the bump over the 5i. On the other hand, the 5i, would certainly be a better choice than many similarly priced options. I felt it was just as enjoyable as the XS, just not as detailed. I really could see some people choosing it over the others... They all three really get to the heart of the music.