Depends on many factors machan. Regardless of the card you get always keep in mind that you are using your money and that it is not some free shit God gave you. Many forget this and keep spending like hell till they cannot pay the bill. Always keep note of what you spend. For instance use a app like Spendee to track your daily spending so that you know how your card is been used.
Then you need to think about the bank. The offers depends on the banks. For example Sampath/Combank give good deals for house hold goods (keells, cargills etc)/clothes and hotels. BOC is know for clothes related deals. Then HSBC offers are very complex (you spend this much for these goods and then you get this free). So you need to choose wisely. Same goes for easy payment schemes at places like Abans/Singer/Softlogic etc.
Another thing you have to consider is the Joining fee and annual renewal fee. Sometimes these fees are way more than the benefits you get out of these cards. So need to choose wisely. And depending on your job/salary you should be able to negotiate a good deal.
If you do the above properly you will find it very convenient. But if you do something stupid this will be a trap that you will find very hard to come out from.
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