I own both and they are both pretty fantastic, but I prefer the sound of Abbey Road to BBCSO. BBCSO was recored in Air Studio if I’m not mistaken where ARO was in…Abbey Road One, which gives it that classic film score sound that I personally love.
That said, ARO is only an ensemble library and doesn’t have legato articulations which is it’s biggest drawback and doesn’t have as many instruments as BBCSO. BBCSO is a full orchestra with really good legatos for all the instruments, but it has only one mic mix and LOTS of reverb, which makes it hard to blend with other libraries.
I am currently doing a rearrangement/re-orchestration of a piece by Yuki Kajiura and I started out using all BBCSO. It all blended together well, but the violins were too quiet even at full modulation, so I ended up switching to Cinematic Studio Brass and Woods and celli/bass, but used Audio Imperia Areia for the violins/violas.