Don't know much about airtel plans but i have switched over to Vodaphone pre-paid.
Bought lifetime validity sim (actually it is valid for 6 years!)
Bonus cards:
Rs. 199 = 1000 minutes (approx 16 hours) free to local vodaphone mobiles, valid for one month, no calling window, 1000 mins are free 24x7.
Rs. 35 = 350 SMSes Valid for one month.
Rs. 55 = 1 Paise per second ALL Local and STD calls! valid for one year! (great for short conversations!)
If airtel can beat these schemes i will switch over again!
Not to aircel though, it is not upto the mark.