Thank you for your opinion, matty
Well, in theory this sounds logical and I agree to you. But I am not made of money, sadly.

The top-end 13" was nearly as much as I could spend at the moment. But if you really think, the 15" is a way better investment, I will try to get some extra cash.
Of course, I agree to you that a computer is an investement with which you will work and live for a couple of years. Seeing it this way...
Again, my only thoughts go to the portability, which I think are more restricted with the 15" than they are with the 13".