Is there a huge difference between them?
The asus is a triple slot cooler and is much better cooler than the XFX one. I believe that the Asus will not be voltage locked whilst XFX ones usually are meaning that it will overclock MUCH better. Other than that. No differences. Depends if all of that is worth the extra £23
*Edit* Actually, the Asus is clocked higher out of the box and has their super alloy power. Probably means a cleaner power delivery to the card.
I won't be overclocking, but which would you pick?
I would still pick the ASUS. It should have higher quality components. It will run quieter, and cooler. It doesn't matter if you're overclocking or not. Always pick quality.
XFX branded PSUs are really good. But their GPUs, not so much.