Deciding about whether a prepaid or a post post will be a good option is very difficult.but there are a few points that can be considered
in favour of post paid
if you have enough money so that in the end of month if the bill is higher then you thought you can manage
you need perfect connectivity and no hassal of recharging it again and again
if you make lots of STD calls because these days STD from cell to cell is only 2 RS thats cheapest than any other medium
if you are a rational person and know your usage and does not need to be tracking your own expenses and can use the service prudently
if your outgoing is more than incoming
for prepaid
if you need phone only for incoming and be in touch
if you are not prudent enough and need to track your bill
if you make less outgoing and more listen to incoming
if you have a fixed income and can not manage the load of a big bill at the end of month
to summarise I can say that there is no perfect way of getting to know which will be best for you what you can do is get a prepaid first and then use for one month and then depending upon your usage pattern decide upon the plan you want to take that can be the most suitable and economical way because these days prepaid to postpaid conversion is free of cost.