There’s a good article in today’s Washington Post on what makes laptops different, and what makes them the same. I agree with most of the tips in the article.
I would only add that for some people the extra weight will be worth it to get a 15″ or more screen. You should consider this carefully, however. I can tolerate a smaller screen than my wife, for example, though some people will tolerate the extra weight for a 17″ wide screen.
If you buy one of these larger laptops, however, you will find them hard to use in airliners. I prefer my 11.1″ tablet for airline use to the otherwise much nicer 15″ that my wife uses. She doesn’t use it on airliners, however.