united economy plus( or extra?)

Suggest looking at www.seatguru.com to view cabin layout, seat dimensions, etc before you commit...just so you know exactly what you're getting for the extra cost.
That said, for long trans-pacific flights we generally found the added comfort to be worth it - better chance of getting a reasonable night's sleep anyway.

We've flown United in the economy plus seating on lengthy legs. It's worth buying the extra space and my opinion is that the 787 plane is an improvement over other long-haul jets as to lighting, cabin humidity and air pressure.

How tall are you? How long are your legs? Personally I would not pay for economy plus for the D.C - LAX leg. I would consider it for a longer leg. I've used economy plus to fly to Europe, but more often I have just flown regular coach and it has been fine. That said, I'm vertically challenged.

At 6-2", it is more than worth it. Even my 5-4" wife likes it though she gets by in standard economy fairly well. It is amazing how much different four or five inches make these days.

Lots of good advice above. I'd stress: you need to go on seatguru.com and check the seat dimensions for each specific flight you are considering - you can not assume that, for example, a 787 is a 787 is a 787 - they are NOT all the same, and they vary significantly! Every airline specifies the seat dimensions for their planes - some put in 9-across seating while others put in 10-across on the exact same airplane. These details will make a huge difference in your experience - especially in a 14-15 hour flight to Australia (that flight makes flying to Europe seem like a short, easy hop).

The 787 (generically) does have some nice benefits (primarily better cabin pressurization) which does help on long flights, but you need to check seat dimensions for your specific flight - the seat trumps everything else on a 15 hour flight.

Note that your 14-15 hour flight across the Pacific comes after a trans-continental flight from the east coast (and 3 hours in LAX, not an airport I enjoy), so your tolerance for misery will already be worn down a bit before you even get on that plane.

By all means, do check the details for the Qantas flight suggested by MrsEB above - according to seatguru, the premium economy seats on that A380 flight feature 19.5" seat width and pitch (space between your seat and the seat in front of you) from 38-42", which is pretty generous. Can United's 787 beat that? Seatguru says no: economy plus on that UA 787 has a 17.3" wide seat with 35" pitch (the Qantas A380 seat wins easily, more than 2" wider and 3"-12" more leg room). If the prices were anywhere in the same neighborhood, I'd certainly go on Qantas.

Do your homework now, or prepare for the consequences. 15 hours in any airline seat is a long time, so you'll have plenty of time to think about your choice...

Thanks to all! We are committed so what will be will be. Don't discount alcohol! =]

Regardless of carrier, for long flights the extra room has been worth it for us, and we are not big people. Warning: you'll find it difficult to fly anything less again. I used to use 5 hours as my cutoff, Then it went to 4, and keeps getting shorter. Now I'm having trouble tolerating my short hops on Southwest! Spoiled!

I'm 5'1" but my husband is 6'3". It is not necessary for me but it is really much needed for my husband to have that extra room!