I think there is an issue in comparing different planes on different airlines as it becomes hard to decide whether the differences you note become related to the make or the airline.

the two planes also target different market: the 788 in particular is a much smaller plane, more 767 like, and the A350 has a much longer range than the 788 though not the 789.

Personally i much prefer the A350 to the 787 like for like. It feels quieter, more spacious and relaxing, somehow air quality feels better even though it is supposed to be similar, and I like some of its standard equipmemts better. The 787 was briefly fun and together with the A380 and A350 has much better air quality than all older models, but in my view it never lived up to the Dreamliner claim, and the novelty of electromagnetic windows (which actually aren't that great) and larger windows wears out after the first flight.

of course, jl which has the 787 y in its originally intended 8 across version shows why people thought it might lead to better flight comfort. But beyond that, in any case, if taking plane into account at all, I'll choose a 359 or a 388 over 788/789 any day.