Go for a diesel car as it would be evil to install CNG in any of these cars while they are new :) Plus with diesel you will not lose out on power and performance.

The SX4 among the list that you have given, comes with a company fitted CNG option - the SX4 Green. Check it out. If you are primarily going to be using it in Delhi, CNG should be fine, but if you're going outstation - then you'll have to switch to petrol for a part of these trips. In that aspect, a diesel would be better - which would then rule out the Honda City from your list. Then it's a question of which diesel - SX4 diesel, Linea diesel or Verna diesel. Verna is the most powerful of the lot. SX4 the cheaper of the lot and Linea the most comfortable of the lot. :-)

Agree with Rishabh. Go for Diesel. Diesel comes close to CNG in economy I believe. I personally think CNG tech is just not up to the mark yet (compared to petrol and diesel tech). It comes close, but not perfect yet. For peace of mind, I would suggest diesel.

1000km a month does not really justify a diesel. Check out the SX4 green. This car retains a usable amount of boot space even with the gas cylinder. Running costs on CNG are a good 25% less than diesel. A tankful of gas will give you a range of around 200km. They have now opened CNG stations in Agra and Gwalior too. Personally, I am a rather satisfied CNG convert. Just stick to regular service schedules religiously and you will be fine. Then there is this satisfaction you get in having your tank filled up for 300 bucks :D In fact, given the rapidly increasing numbers of Honda Citys, Accords and Toyota Corollas I have been observing queued up for gas, I think these supposedly hi-tech engines run just fine on gas as well.