I'm sure both games will sell great and make the respective Stern and JJP fans happy. The themes are so different that I think most people will be naturally attracted to one or the other, making a real head-to-head comparison difficult. Watching from afar, I liked the flippers and ramp flow of Wonka, but I didn't like the art, video, and sound package. I think it has better flow than Black Knight. But, to put the Wonka theme in my house at its price point, I needed to be blown away and I wasn't. I do give JJP kudos for getting their entry level game price down a bit.
I've been very critical of Richie phoning in his designs and art packages these past few years, IMHO of course. However, in the case of the new Black Knight, I was pleasantly surprised, though I am a little concerned by the number of bricked shots I saw in its Deadflip stream. If I had to make a decision today, I'd take a Black Knight Pro based on theme, price, and sound/video/art package that I've seen so far.
Edit: I'd really like to see the Wonka playfield layout used again with a different theme. I really liked the ball movement on it.