From What I've Read Here (i'm Also Very New To The Board), It's Better To Be In A Niche You Like Because You'll Write About It Passionately, Therefor Subscribers Will Come. I've Also Read It's Better In A Long Term Sense.........
My niche keyword that I ever had such a high CPC is working visa and work permit, something related to legal, law, lawyer. I got 1.54$ per one click ^_^
I feel that it doesn't matter what niche you go after as long as you optimize, Monetize your site(s) in a popular niche or a niche you like all you really need is 1% of the market to be well of...
I prefer niche...because it's something that you're (as previously stated) passionate about, you may enjoy writing about, and moreover you can see SERP's move a LOT quicker than trying to rank for "lawyer"...so you're more inclined to continue working and growing...instead of seeing N/A on every line of your intended keywords on DP's tracker. That would be depressing =).
i prefer to the niche i like coz that means i could write more into it and subsequently i could find my readers and the more readers i have the more adsense $ i earned