The Ertiga is compared to the Innova because that's what they have in India. Indeed, against the Innova, it's not very spacious.
But it's the Avanza and Spin you have to compare it to here. Yes... the third row is cramped... you have to move the second far forward for appreciable third row space... but the Spin has similar issues
The Avanza still has, I believe, the advantage in terms of third row seating and cargo space (third row flips forward, leaving a very deep boot.)
But the Spin's sliding second row gives you greater comfort there than in any other small MPV (except perhaps the APV)... and the seats feel more substantial than the Avanza's. Yes, the way the third row folds kind of sucks, but then you could do as I do and leave it in place and dump the bags on the seat.
The Spin has more durable feeling seats, but as I've said elsewhere, the high seat point is not great for shorter passengers.
I'm thinking the Spin is the best casual use family car of the three... very car-like, very comfy... the only major... MAJOR issue, really, is the
beige interior. I'd get the base model or the mid-range model and spend the extra on some leatherette seat covers, door panels and trunk-liner.