First and foremost, I'd choose the topic in which you are most interested. Writing a comment sucks more than anything I've ever done, and if my topic hadn't been compelling for me, I don't know if I would have finished. For the record, my comment has nothing to do with any specialization or area of law I'm very interested in; it just happened to be an interesting topic.
That being said, etting published is all about the timely, big-impact issues.
I'd say which topic you want depends upon what you want to do after graduation. If you want to clerk, especially at the federal level, then I would choose the topic you think will get you published.
If you don't care about clerking, then it won't really matter if you get published. It's definitely a nice honor, and looks awesome on the resume, but that's about it. In that case, I might be more likely to choose the topic in the field I was interested in, espeically if the area of law was small or competitive. It could be another way to show interest/commitment to the practice area.