It is tough to decide between the two cars. I think that, for rear seat comfort, WagonR is better and gives a very airy feeling. Plus, the car is nice to drive in the city with excellent visibility, good engine response, decent gearshift, and a quick steering. The ride is also composed and the car is pretty stable in the straight ahead position. It rolls only due to high centre of gravity.
The Santro is a more fun drive with light controls, a snappy gearshift and a very peppy engine too. But on poor road surfaces, the unsettling feeling of the Santro gets very irritating. However on a good road, it is pretty stable and secure, and handles well too. Both cars have good brakes, but Santro's a bit grabby. Both cars desperately need a tire upgrade.