avichandana20000, I think you relying upon wrong factors for making the decision. Nobody buys an external HDD for performance. Obviously you won't be using them as your OS drive or installing software...they will be used as your storage. And the performance difference between a 7200 RPM HDD and 5400 RPM HDD is really small in real world scenario.
You have to think why you need it:
You have huge amount of data and you need more space for storing them. You don't need to carry them all the time, but sometimes. So here the most important factor is its storage capacity. In this case a large amount, say 1 TB 3.5" external HDD will be ideal solution.
You need a device you can carry as you need to share data with others for some purposes. Also you want to be connected with your data on the go...while traveling and using your Laptop. Storage is required but not that much. In that case a 2.5" portable one with capacity of 30 GB or 500 GB is a very good choice.
See, the choice should be based on your type of usage, not by the performance of external disks.