If the aircraft has sleeper seats in first, there is a big difference. I find that being able to lie completely flat improves the quality of my sleep enough that I'm actually able to function on arrival and get some work done. The food service is better (restaurant style), with more choices, crystal, china and silverware, with no plastic serving trays (much better choices of cheeses, deserts and liquors). The ratio of flight attendants to passengers is less but with so much time it really doesn't make any difference. If I had to pay for it from my own pocket, and it's for leisure travel, it's not worth the difference (unless you have a very elastic budget).