I'd pick the 340 over the 787 purely because of the seating config. As you are travelling as a couple the readily available 2's courtesy of the nicer 2-4-2 layout of the A340 would win for me every time. I ummed and aaahed over the extra legroom seats on my last VS 340 flight and didn't bother in the end - I was glad I didn't as frankly the regular pitch was 'sufficient' and I'm 6ft.

TATL Economy as couple on a plane configured 2-4-2 really is quite bearable.