It depends how much you value portability. A 15" laptop will always be much more portable than a 17" - and, in many ways, 17" is something of a compromise in screen terms anyway. I think, based on what you say, I'd go for the smaller screen size laptop.
Don't worry about the external monitor. Anything you buy today will be good for a long time - there aren't that many innovations taking place in the monitor market. You'll get it cheaper in the future, of course - but that's the technology market for you.