I'm personally partial to the Latitude packs- the single zipper and from the bottom makes the Nimbus packs less versatile in my eyes. But that's mostly because I like panel loaders.
They don't make the Latitude Vapor anymore- or, for that matter the non-Ki Nimbus Latitude. As far as I can tell, the Meridian packs are the Latitudes' successors.
The Latitude Vapors are getting harder to find in anything but short, though they still have quite a few at the Granite Gear surplus/outlet store- PM me if you have any trouble finding one in your size.
A few numbers which may or may not help…
Latitude Vapor = 2lb 10oz
62 L, 70D ripstop + 210D Cordura
Meridian Vapor = 2lb 14oz, w/o lid = 2lb 6oz
52 L, 100D ripstop + 210D Cordura
Nimbus Meridian = 3lb 8oz, w/o lib = 3lb
60 L, 70D ripstop + 210D Cordura
A lidless Meridian Vapor does come out lighter than the Lat Vapor, but with some loss of volume. You do end up getting a tougher base fabric than the Nimbus Meridian or Lat Vapor in return.
Aaron