Go take the Nissan UDs. I confirm you will like the suspension.
Mind you, SG road are rough? U sure anot? Our roads have one of the
highest maintainance you know? Those bumps and cracks that you see
are just very few deformities... Our roads are so much better
compared with other countries, and we should count ourselves lucky
for that. 
Actually you are right, conventional suspension are cheaper and
easier to fix. But however, if alot of our buses uses conventional
suspension, confirm got people complain suspension is cranky, sit
until butt very pain, not soft. 
Like you said, you've enjoyed riding on B10TLs soft suspension. So
why not make other buses having air suspension so that other people
can enjoy the comfort benefits of air suspension?
Back to B9TL, B9TL also have this feature of electronically
controlled air suspension.. I've sat on one B9TL rego SBS7375Z on
198 and while the bus stopped at a banked road, the air suspension
automatically shifts the bus back into level position. Thus the
side that im sitting on ( im sitting on the left side) goes up,
while hearing the sound "shhhhh". Cool.
