Hi,
I will recommend you Spark! Now points you mentioned:
1. Fuel Economy: Spark, Estilo, Santro (If i10 ..well, Generally i10 will give you upto 12kmpl with a.c. in city).
2. Spare Parts: Every car is almost similar as far as spares are concern, I have checked the prices its almost similar, maintenance of every small car is more or less similar. Comparision of part prices.. Spark & Others? look here
3. Driving comfort: Test drive all 4 and see in which car you are most comfortable and in which car your family is more comfortable at rear.
4. A.c., Power windows, check out Top model of Spark .. Spark LT, the best Value for your money. Boot,.... well boot is not good but you will get 60:40 folding rear seat to carry more luggage occasionally.
5. Height of Driver, Test Drive every car to know in which you are comfortable, I used Santro and wagon-r and now in small, I am using Spark LT, I am 5'8" and I am comfortable in Spark. Wagon-r is the best for tall people.
6. Budget: In delhi you will get Spark LT @3.75lacs on road including Cashless ownership offer (You need not to pay for next 3 years/45000 kms.).
7. Resale value: You will definitely get good resale value from Spark too, I checked it and the differenc between Wagon-r & Spark with same model and similar running was around 15000/- which is not too much and specially the price difference between both. Also, I would like to add one more point, Now Spark is selling in good no.'s too, At orkut I have seen today that last month the sales no. of Spark was 4390 units (The best after its launch), that shows that car is selling now coz its more than a year now and No- Problem car. So, in future I am expecting that resale will be better.
Now about i10, hyundai is providing the best interiors in i10, best in class and definitely better than few premium hatch and entry level sedans but other features, well... you can get better features in less price if you will opt for Spark, Spark is Value For Money!
People dont change when you tell them that there is a Better option...
They change when they realise that there is No Other Option..