I think it's quite clear that West has generally higher level of pollutants (in 2011) based on the NEA website link from Chestnut. But the difference will not be as significant as the levels in certain countries described as cancer-causing. It should still be within very safe ranges as the human body has the capacity to deal with a certain level of pollution.
PM2.5 levels could also possibly be lower in the West. It all depends on how the survey is done, what industries or activities produce them, wind directions, seasonal changes etc etc...
It is pointless arguing over specific locations and projects. Rental demand and prices are determined by a multitude of factors. For each his own. Each believe in his or her own area (even abroad) will do. No point condemning others for their choices.
The three laws of Kelonguni:
Where there is kelong, there is guni.
No kelong no guni.
More kelong = more guni.