They are both anemic. .Biz has a longer history as a relative failure. .Mobi is merely a few years old and therefore can claim youth and immaturity as reasons for failure to date.
Don't try to buy and flip either. Domainers don't have cash now, and that is part of the problem with flipping today. End users are still mostly unaware of anything other than .com, .net, .org, and some ccTLDs.
Registrars and registries have made enough $$$ from domainers so as to not be motivated to market to end users. When Godaddy spends millions for Super Bowl ads, you don't hear a peep from Godaddy about .biz, .us, .info, etc.