Just as with stocks, the smaller the market cap, the more gambling you are doing. Smaller caps require higher amount of due diligence and you're also often at an information disadvantage with other investors for these projects. As such, it's generally better to stay with those within top 50, delving only past that for cryptocurrencies that are either promising or have a ton of "shilling" support, as regardless of merit these types of cryptocurrencies tend to pump.