First International Flight

I don't think you will notice a huge difference in "space", one seeming much bigger than the other, but seating choice and layout may make a difference.

If I am travelling alone or with my wife, I like a 2-4-2 layout, where my wife and I take the two seats that are the window and an aisle. We then have no one else that we are bothering if we need to get up. When we traveled with a kid, we then looked for a 3-4-3 or a 3-3-3 arrangement, so again we were all together. If you are a bit claustrophobic, then I suggest getting an aisle seat and maybe upgrading to Economy Comfort or a Bulkhead seat.

Go to seatguru.com and plug in the two flights. They will show seat maps and give you information on the seats themselves--size, amenities, etc.

Then you can decide which you prefer.

Personally, if I could, I would book the Economy Plus that is generally available with United. That is worth the extra money especially if a little claustrophobic.

Yes, the 767-400 is preferable to the 757-200, for an international flight, because the 767 has 2 aisles and the 757 only has one aisle.
2 aisles makes it easier for you to walk around and stretch your legs, whereas in a long flight on a 1 aisle aircraft like the 757, the single aisle is more often filled with flight attendants, food and beverage carts, and other passengers, which makes it more difficult for you to get the blood moving in your legs.
Go to seatguru.com, choose United and the type of aircraft (you don't have to put in the flight #), and you'll see the difference.
This is a potential health issue for some of us older folks, who need to avoid sitting in one spot for hours. 2 aisles makes it much easier for you to walk, you can even do circles on a 2-aisle aircraft, and the flight attendants won't crab at you to get out of the ailes, the way they are likely to do when there is only 1 aisle.

Another advantage of the 767, if a couple is traveling, is that United's 2-3-2 layout on this aircraft makes it possible (book early) for a couple to get one of the double seats, so you don't have anyone else in your "row".
The 757-200 has only 3-3 seating, so a couple will likely have someone else in their row that they have to inconvenience to get up.

There is no doubt in my mind – I would choose the wide-body plane. With a 757, the only aisle will often be filled with flight attendants, carts, and people lined up for the bathroom. It can be very difficult to leave your seat. And if you are unfortunate enough to be sitting near a bathroom or the kitchen, you're likely to have people leaning into your seat space. Not how I want to spend long stretches of time.

Not usually a consideration going to Paris, but coming back to the US--if the winds aren't being helpful--a 757 may need to make a fuel stop on the way, allowing you to add someplace like Bangor or Gander to the list of airports you've visited. More importantly, if you have another flight after your transatlantic leg, you might miss it do to the increased time that a fuel stop entails. With the 767, not an issue.

I agree. And go for the extra-legroom upgrade. I always try for a bulkhead aisle seat, for me it's the difference between feeling like a sardine rather than an anchovy.

Taller travelers may find the window seats in a 757 a little too cozy. As a narrow-body aircraft, its curved ceiling gives (me) the sensation of being closed in. And I am certainly not tall. But to save a pile of money I'd buy the ticket and hunker down.

Thanks to all for your very helpful suggestions! After studying seatguru, I booked my RT ticket and chose premium economy aisle seats. Now looking forward to the trip!

dede,
Which aircraft type did you book?