Try and develop something you know alot about,

Also for adsense, stay away from webmaster/seo websites as most webmasters do not click on your ads

Based on adsense,

If you have 10,000 Webmasters visiting your site or if you have 10,000 non webmasters visiting it's the "non webmasters" that will make you the money (IMHO!)

And i can honestly say directories (general) have to be the worst possible niche for adsense, unlesss you can develop a niche directory relying on targetted traffic.