Go for an Alto without any doubt. It cant be compared with a Nano or Eon. Alto has proven itself for its reliability, FE, user friendly nature and so on. Whereas an Eon or Nano, though it may be a new product, what about the reliability and VFM ?
Nano, despite of its good FE, its a twin cylinder one. While starting it reminds a truck. Its not at all suitable for long drives. About EON, its a Hyundai product and its maintenance costs will be definitely high when compared with Maruti.
So go ahead of the Time tested Alto.