No upgrade out or back. (No refund of my miles yet, either, but I know that'll take a while.) I was in 31E on CX. The seat was worn out and no wider than on CO, and as mentioned, the seat pocket in front of me had a torn bottom. Everything slipped out onto the floor at my feet. And the TV monitor, while bigger than CO's and having more channels, was horribly fuzzy, a quality like something out of the 60's with rabbit ears! But I will say CX's seat configuration was better in my opinion: 2-4-2 instead of CO's 3-3-3.
Different also was CX's flight path. It flew over Taiwan, skirted Japan, Alaska and the Canadian border and then swooped into New York. Isn't that farther than the polar route? I also noticed the CX plane going 640 mph a lot of the way. CO was going about 100 mph slower.
As for the fish odor being worse in summer, I can't imagine! It really cast a pall upon an otherwise sense-expanding experience! But, as I said, it was a minor drawback. A small pall if you will.
But CX's A346s are pretty new. Delivered around 2002-3, they're a bit newer than most of CO's 772s.
The eastbound flight path HKG->EWR/JFK depends on where the strongest tailwind is. Sometimes it takes the southernly route you get, sometimes it crosses NE China, Russia, and then into Alaska. My last trip used the Polar 4 (not the Polar 2 on the westbound) that goes north of Alaska - very very rare.
772s fly faster than 346s. The eastbound is always faster due to tailwind. For westbound, today's CO99 is scheduled for 14:42; CX831 is scheduled for 15:06. Both depart about the same time, and the difference in flight time from EWR and JFK is negligible.