Re: Which laptop I should buy for longer use........
Well, if prices in India are the same as they are in the USA for the various Apple laptops (I really don't know), it looks like you have several options. My understanding is that in most tests the 13 rMBP is still considered the sort of king of battery life. That said, the 13in MBA is very respectable, and if you need the power that the 15 rMBP has, it can also be respectable with non-intensive tasks, though as soon as you are pushing it with gaming or some sort of export/render, while it will be much faster than the 13 in, it will also go dead much faster as well. This all is based on the guess that you are referring to the battery life.
If you are looking for the device that will last the most years for performance, that is easy, buy the best you can afford, specifically RAM, as the entire line up of Apple laptops other than the quite old non-Retina 13in MacBook Pro can not have the RAM upgraded after purchase, so you are going to want to get as much as you can at time of purchase. Personally, if I was trying to ekk as long of a life as possible out of a MacBook, I would go for the top of the line 15 rMBP with the 16GB RAM, dedicated GPU, and the i7, though I would likely skip the Storage upgrade, as the standard 512GB SSD is way more than I personally would need, as I use external storage for as much as I can, and while RAM and processor needs may increase over the life of the laptop, the Storage needs are going to be based on your use, not the needs of the system when you consider it in the limitation of the life of the Laptop.