With regards to the mesh:
Most global meshes are using the same source data (NASA SRTM + some local mesh data e.g. for some parts of the alps). These source data have a lot of voids which need to be filled by interpolation. Depending on the method used the end results can look different, but in general I would not say there is a global mesh that is better than another. I have FSGlobal2008 and am quite happy with it. In most parts of the US and Europe it has the same resolution as the FSX default mesh but for the rest of the world it is a great improvement.
If you are interested in very high resolution mesh for smaller areas, I can recommend FSDreamscapes:
http://www.fsdreamscapes.com/web/As for the airliner, in additon to the PMDG ones, I can recommend the LevelD 767:
http://www.leveldsim.com/sevensix_home.aspIf you are interested in a vintage airliner, the A2A Stratocruiser + AccuSim is the state of the art:
http://www.a2asimulations.com/Best regards
Manuel