Ask yourself how many hours a month you would put 2 adults on the 3rd row of Ertiga ??
Based on the answer go with Ertiga or Innova.
I am an Innova owner who has driven the Eritiga a lot. Honestly I feel Innova is old technology. Toyota has put in an undertuned commercial engine in MUV (mini van) that makes lot of noise. The gear stick is like a commercial one wrapped in leather. Having said that, it's the best vehicle in India today (under 20L) if you have to haul 7 people. High quality!!
Ertiga is also high quality, better engine, better mileage than Innova, excellent ASS, drives like a car (gear stick is like a joy stick), super handling, super braking, excellent interiors, loaded with safety features, much cheaper than Innova, etc., (I can go on and on). Only problem is 3rd is a bit tight.
Once you answer the question you'll know which one to buy.
Note: If you plan to put just 1 adult on the 3rd row then you'll have a harder time justifying the need for Innova.