I talked with Simon Spanton (editor at Gollancz--Brandon's UK publisher) about this cover a few weeks ago when I was in England.
What he said would essentially confirm your question. He didn't want the cover to scream only steampunk like the McGrath cover does. He told me steampunk is a bit more of a niche market there, so limiting it in the way the McGrath cover potentially does could hurt the sales. This way, it's open to interpretation as the front of a train, or perhaps something else. I personally like this cover (with the exception of the horrid tag-line) much better...though part of that may stem from my growing dislike of the identical feel of all the McGrath covers--to me the US cover is just like the Vampire Empire novel just on a different area of the color wheel. All McGrath's stuff says "I'm Urban Fantasy. Open here for emo vampires."
Again though, it's all subjective. For example, I really dislike the US WoK cover. It isn't even close to the awesomeness that Whelan can achieve, in my opinion. As my favorite saying goes, different strokes for different folks.
Edit: For Brandon's Elantris, The US cover is amazing. The UK cover is SOOO different, but I absolutely freaking LOVE it.