I have a 231x as well and they are not very similar. The 235x is quite a bit larger. About 25% longer and has a much better pocket structure in my experience. The shape seems different as well and I find, probably because if the extra length that it is easier to pack the 235x. That all said, I used to use the 231x a lot as a second bag on top of a roller but haven't used it since I downsized my packing list. I should revisit it as a main bag.