Based in HK so I fly CX most of the time including a fair bit on their longhaul J. Just flew HND-SIN and got to try SQ J for the first time on their 777-300ER. Pretty interesting experience for comparison.

Link below is CX's longhaul J seat for those of you not familiar:
https://www.jetphotos.com/photo/7310686

​​SQ has a wider seat in non-reclined or slightly reclined position compared to CX. Overall I find the seat very comfortable while in upright position. CX's seat also good in this regard but I have to give more credit to SQ due to the greater width. Very good if you're simply watching TV or doing some work and not trying to sleep .

Once you recline the seat considerably however, CX wins. I didn't like how the foot well is located to the side on SQ's seat. I find myself having to sit or lie down diagonally across SQ's seat when reclined by more than 50%. Unless you sit on the seat diagonally it's impossible to extend your legs . I had a window seat on K aisle so my legs had to point to the right when significantly reclined. Feels awkward when your back has to angle to one side even though the seat back cushion faces forward.

Bed mode: Almost thought seat couldn't convet into a flatbed until I did some research with inflight wifi. I bet a lot of people unfamiliar of the seat wouldn't even be aware you have to get out of your seat and flip the seat back cushion forward to convert into a bed. Would be helpful if they placed a manual in each seat for reference. Once again didn't like how you have to lie diagonally in the seat. I am 175cm and still able to fully extend myself but I can imagine it could be an issue for taller folks. Also since the foot well isn't particularly big there aren't many different positions you can take while lying flat.

Some might find it cumbersome to have to get out of your seat to change in and out of bed mode, particularly if you wake up just as a meal is being served.

Amenities and F&B: SQ wins. Particularly like their offering of slippers which is a nice alternate to your own shoes while seated. Food is not bad on CX but nothing to write home about; SQ however was better. I particularly liked their chicken satay for appetizer.

Storage space: SQ wins. More storage space for small items and backpack which could be stored below foot well. On CX J I always have to store my backpack in overhead bin.

Wifi: Complimentary wifi for J pax and elites was nice. Hope CX would follow.

Considering all above factors I still prefer CX's J, mainly due to seat comfort when reclined or fully flat position. Not exactly a fan of SQ J's foot well located on the side forcing you to sit or lie diagonally.