Multiple choice for multiple answers. English translations are below.
Which lyrics do you prefer?
Both.

alan sings of love and war, while Ayu sings about not giving up and looking forward to the future.
Which song has better vocals?
alan - RED CLIFF ~心・戦~. I've heard this ballad live and it stills sounds epic. Its so emotional it almost brings me to tears.

It honestly gives me chills. Ayu's voice sounds nice in this song, but not overly moving.
Which song do you prefer overall?
alan- RED CLIFF ~心・戦~ I like the composition and background instruments, not to mention alan's wailing in the last part. Her voice is really enchanting and haunting at the same time, and goes well with the theme. I did like the composition of RED LINE ~for TA~ though, its a nice refreshing sound to hear from Ayumi. Ayu's voice in this song brings a smile to my face, while the song is uplifting.

RED CLIFF ~心・戦~

RED LINE ~for TA~
Spoiler:
Translated by Nukaru
We turned our tears into just one prayer
The miracle we saw on that day in that instant is no illusion
However I wonder if it’s sad to be swept away by the ordinary
Try to remember, if you’re in despair
"On this day when you have decided to give up
somewhere, someone didn’t want to give up."
And remember that I am tightly, tightly
holding your hand and the future
Thus I want to face another sunrise
Please don’t be afraid of taking a step forward
Try to remember, if you slam into a wall
"On this day when you seem to be losing
somewhere, someone wanted to fight."
And remember, no matter how far away
your hands are connected to the future
Try to remember, if you’re in despair
"On this day when you have decided to give up
somewhere, someone didn’t want to give up."
And remember that I am tightly, tightly
holding your hand and the future
Try to remember, if you slam into a wall
"On this day when you seem to be losing
somewhere, someone wanted to fight."
And remember, no matter how far away
your hands are connected to the future