If someone says "don't buy ecotec", do ask him why...? if he says its useless... he is a big fat liar.
I think ecotec is the best thing thats ever happened to Corolla+CNG. They are using the SIS type advanced CNG kits and a dedicated ECU for CNG. Buy this kit from the market and without cylinder its gonna cost u 80k+.
The amount 125k Rs is totally worth it, however, the reason why toyota insiders suggest u not to buy ecotec is from an unconfirmed source that ecotec corolla is not VVTi. Its the simple 2nzfe cz vvti+cng+60k kms on the odo= epic failure...
Cng screws the valve train in most cases...
So, if you want fuel economy, go for ecotec... drive it the way u want... floor it... you'll always be saving bucks on every drive... 
And on a friendly comparison b/w xli genuine petrol an an ecotec gli on cng, the ecotec was actually a second faster... 