I like them both but I have to admit I prefer the US cover. Before I had seen the US cover I adored the UK one but the US blew me away. Firstly the patterns on the UK are nice but take up a lot of space along with the LARGE title so everything has been very squashed up. Also the half vampire face thing has already been done and the colours are just very dark and dingy. What I do like though is the blood splatters. I think it adds a lot of charecter. The US is vibrant, spaced out, better fonts, more eycatching and better colours. The fonts are much better and the blue stands out! Also I love the way they put the house there glowing. The only thing that lets the US cover down is from neck upwards (including spiky the necklace) I dont like the model with the spikes, piercing etc.
Overall I have to go with the US just for being better as an alrounded cover. :)