I love my BK. It's a true battle...100% adrenaline and as brutal as they come. The art and A/V assets are awesome if you are into fantasy and metal. Tons and tons of personality and awesome characters.
But...you are putting it against one of the greatest games ever made. Maiden definitely has a more interesting layout and deeper rules. More to shoot at, more to do.
Tough call. Most people are going to tell you Maiden hand's down. I personally (slightly) prefer BK, but I know I am in the minority (and to be fair...I haven't owned a Maiden, yet). Since you already have Maiden, you know what you are getting there. If you are done with it and ready for a change, go for it.