Opinions on this. Using adsense, would you rather 30 blogs with little income, or ONE blog with high income?
One blog with high income. If you have one blog, you'll have more time to update it with fresh content and more time to promote it and SEO it.
One quality blog will do better in the long run than 30 krappy blogs.. It takes a lot of time and effort just to have one good blog.. trust me, i've been trying for over a year and some say i still don't have a quality blog..
If you mean many blogs with total crap, ofcourse answer will be - make one good blog, don't waste your time for trash. But if those 30 could be done with a little effort, to keep at least some quality, this could be better option and give you more stable position.