Many discussions about Baidu (NASDAQ: BIDU) focus on its slowing ad revenue growth and its rising investments in online-to-offline (O2O) services. The bulls think that the lull is temporary, and that the expansion of its ecosystem will pay off. The bears think tech giant Tencent (NASDAQOTH: TCEHY) could overtake Baidu with its growing ecosystem of apps, services, and games.