A gas convert

I HAVE had my 2003 VY Series II Berlina wagon booked in for a gas conversion for nearly four months at my local country dealer. I have been told the long delay is because no doughnut tanks are available in Australia. I was also told the only conversion available for my car was the old gas mixer conversion at a price of $3200. But when I phoned another company I was told they could do a conversion in two weeks and could do either the gas mixer at $2990 or the sequential vapour injection conversion for $4700. Would I be better off to go for the more expensive conversion.