I’ve never been on either but based on my quick skim on these two articles, I’m in two minds about both options. The BA cabin seems rather crowded in a 2-3-2 and 2-4-2 seat configuration and is a slightly older but functional design. BA also has the alternating front and back facing seats which is novel or strange
Iberia seems to have 3 different A350 configurations but they seem to be all in 1-2-1 seat configurations - which is a bit more standard (modern).
With no clear winner, I’ll probably go with the one with the better connection/flight time or just try something you’ve never tried before, e.g. BA for it’s novelty.