The difference in price ex-showroom Delhi si around Rs.45,000, so to come to any conclusion as whether one should pay this additional price for ZDi over VDi. Being the same car, overall they will behave quite similarly in terms of drive experience.
ZDi gives you 4 more speakers, alloy wheels over steel wheel rims in the VDi, rear wiper and wash, Dual front airbags, ABS, EBD, brake assist, rear defogger, features that make eminent sense in today's road condition. Even if one was to get these fitted from after-market you would be paying similar amount or just slightly lower than Ertiga ZDi's ex-showroom price.
It seems Maruti Suzuki has got its pricing right in this case and getting Airbags fitted from outside will definitely impact your manufacturer's warranty a risk, I would request you to avoid. One does not know whether one can get an ABS, EBD and BA fitted outside, even yes, then too quite avoidable. Only item you can safely get from aftermarket are the alloys, but then they are not worth the pain or additional cost as one can get decent wheel covers and make the wheels look good.
We recommend the Ertiga ZDi over Ertiga VDi
We at www.ecardlr.com always recommend that you define your needs from a car as clearly as possible, go for exhaustive test drive of cars in your consideration set. Treat our inputs as suggestion and not as a final recommendation.