I used a 15" for years, and I'll never use a 15" again. Plugging in an external monitor was a workaround that helped when I was at a desk, but much of the time I much prefer to have my laptop actually on my lap in a comfortable chair or on the couch, where external monitors is not an option.

And it's still way too big for my travel needs. Yes, it fits in my bag, but I regularly have other things I want to take with me too.

So back to my point: It depends on your usage pattern.