Thoroughly research the Alto AGS. I have heard that the AGS (automatic gear select) fails at around 25,000km. The AGS is basically a manual gearbox with a robot that depresses the clutch and changes the gears on your behalf. Not the most reliable transmission on offer. If this was a CVT or auto gear I'd recommend the Alto but not the one with AGS.
On the other hand the Picanto has a proper conventional 4 spd auto box. As per the sales literature it should give you about 12 km/L.
If you were OK with manual car, then go for Alto. If Auto is must then Picanto.