I've always (subjectively) liked Ayu's vocals more than Hikki's, but in some cases - like Sakura Drops or something, nothing but Hikki's voice would work. Which is... how it's meant to work.
This may sound really bizarre, but Hikki's vocals sound significantly worse to me in English. I guess it's because I know what the words are meant to sound like, I can understand how they fit together and work to create the song, how the structures of the words work for the song... and how her voice just doesn't work at times.
Until I listened to her English work, I thought she was REALLY good (the typical opinion of her, basically), but then when I heard her in English I was just like...wow, she's kinda average, she just makes good songs and sounds interesting.
It's odd :/