I've got 1.3L vitz, and have driven 1.0L vitz. 1.0L felt so under-powered and noisy that I had once decided to go for swift, then I drove 1.3L and went for it. And I think it was a good decision.
[TABLE]
<tbody>[TR]
[TD] Parameter
[/TD]
[TD]1.0L 1KR-FE
[/TD]
[TD]1.3L 2SZ-FE
[/TD]
[/TR]
[TR]
[TD]Mileage (City/with AC)
[/TD]
[TD]15 kmpl*
[/TD]
[TD]14 kmpl**
[/TD]
[/TR]
[TR]
[TD]Mileage (Highway/with AC)
[/TD]
[TD]18 kmpl*
[/TD]
[TD]18 kmpl**
[/TD]
[/TR]
[TR]
[TD]Engine Noise
[/TD]
[TD]Mostly Unbearable
[/TD]
[TD]Like any other car
[/TD]
[/TR]
[TR]
[TD]Knocking
[/TD]
[TD]Fuel dependant (Usually there)
[/TD]
[TD]None
[/TD]
[/TR]
[TR]
[TD]Power
[/TD]
[TD]Underpowered
[/TD]
[TD]Good enough for highway
[/TD]
[/TR]
[TR]
[TD]Winner**
[/TD]
[TD]-
[/TD]
[TD]This one
[/TD]
[/TR]
</tbody>[/TABLE]
- As per my judgement from PW members, it varies largely though, some even get 9 kmpl in city or some even claim 16 kmpl.
** As per my experience
And as @levin has correctly pointed out, it depends on other factors too. 1.3L can be more economical than 1.0L.