Hi Rob, :)
I only have the M.Qs, but gather that the Manhattans are more open and almost acoustic in tone/presentation. This is helped / determined by the more subtle Alnico III magnets and 'cooler' windings.
By way of contrast, the Mississipi Queens are Alnico V bridge / Alnico IV neck with a hot winding in both. They are consequently a much more powerful pickup. However, if 'tamed' by picking technique, pickup height and E.Q - the M.Qs are clear, fat and warm - and absolutely purrrr . ( That is how I use mine ). If let loose and hit hard, they are clear, ripping, in your face Rockabilly / Rock 'n' Roll heaven.
For Jazz and/ or Ballads, either work well - but I gather the Manhattans are a more sedate and spacious sounding option. :)
Regarding the absence of Manahattans from the site, I seem to remember being a 'special' order item - and as such are not advertised in the regular way.