for me it is like this:
civic - bigger cc engine in same price and drives good on CNG (i.e. doesnt need constant tuneups).
Altis - have all advantages over civic except smaller engine and CNG needs too much tuning to run (the electronic throttle just refuses to support CNG).
for me i cannot run car on petrol, so i want to put CNG. hence civic seems more obvious choice, as i do not want to go every now and then to adjust the CNG in altis. but there is no doubt that comfort wise Altis wins hands down. Also altis will give better fuel economy on petrol on CNG coz of its smaller cc engine (but the constant drama of visiting mechanics to set the CNG offsets the minor difference). And for 1.8 million Altis i would rather spend 1.7 and get M/T civic.