If you don't need the power of the 15", I'd say go for the 13". You can buy one of those LG screens, or whichever display you fancy, with the money you save for those times you may want to have a larger screen.
I own a 15" MacBook Pro Retina and my wife is using a 13" MacBook Air. I'm not carry my laptop around that often so for me the weight is not that important. I still love the plus on space and the high resolution because my eyes are not the best (strong shortsighted).
But I have to admit the size of the 13" MBA or MBP is a very good compromise if you have to carry your laptop around often. BTW: We aren't using any external monitors on our Macbooks (except for the 40" TV with the Apple TV)
My previous laptop was a 17" Toshiba and when I compare it to my MBP it was a very heavy piece of plastic!
Hope this helps!
My uncle does. He has a 15” MBP. Personally I think a 15 is a little big but a good size. On a lap is not a good idea. I have a 13” and I travel with it a lot. It it’s perfect size for travelling since it is much lighter and smaller than the MBP 15”. I have tested the 15 and I like it a lot, so I am thinking of upgrading my 13 to the 15 next year. Instead of the 15 I may buy the 16 (which is the same size a the 15 but an edge to edge screen).