The one I'd exclude right away would be the 17''. The spec improvement over the 15'' is easily surpassed by the comfort of taking a 15'' with you, over a 17''.
However, between the 13'' and 15'', that's a different story.
As I said, I'm with you. I also do not know what to buy. Even with an external at home, I don't like to be dependant on it for working (I like to work in very different places inside my own home). The size of the 15'' is more uncomfortable than the 13'', definetly, but I never carried around 2,5 Kgs, so I dunno how different it is from my 2.8 Kg Sony Vaio.
That being said, I think regardless of what you choose, you should put 8GB of RAM and a SSD Drive, considering price is not a factor.
However, between the 13'' and 15'', that's a different story.
As I said, I'm with you. I also do not know what to buy. Even with an external at home, I don't like to be dependant on it for working (I like to work in very different places inside my own home). The size of the 15'' is more uncomfortable than the 13'', definetly, but I never carried around 2,5 Kgs, so I dunno how different it is from my 2.8 Kg Sony Vaio.
That being said, I think regardless of what you choose, you should put 8GB of RAM and a SSD Drive, considering price is not a factor.