Obviously sound is subjective, but based on demonstrations we do instore, the Naim's tend to be more popular. This seems to be because the Naim has a bit more get up and go than the Arcam - the Arcam is more suited to those who like things a little more laid back. The Naim, due to it's design tends to have a better current output, which is capable of driving more awkward speakers than it's direct competitors. I personally would prefer the Nait 5i, which as far as I'm concerned is one of, if not the best, sub £1,000 amplifier.
Peter Pan - while there may be beter suited speakers for your amplification, I don't see any problem with the KEF/Naim pairing. I've done a few dems where Nait 5i and XS's have been used with the XQ series with no problems at all. The Marantz isn't quite going to measure up to the Naim.
Chebby - the basic sound of the 5i and XS aren't that different - obviously there's refinements and more headroom, but Naim has a specific sound. I can only assume the 5i you heard was either badly matched to the speakers, or not warmed up properly.
Naim electronics are far more compatible nowadays than they used to be, and don't really find many speakers that won't work well with them - our customers have bought a variety of manufacturers to go with them, but the most popular has been Proac.
James - nobody has asked which speakers you are using, as this could be the deciding factor.
Regards
David