We recommend that you use a hardware wallet, to be the full owner of your coins. While Bitcoin may be the best known, Ethereum has about half the market cap of Bitcoin and is seeing broad adoption. By providing some of the global hash power needed, you receive rewards and get paid in crypto-currency. If you were to build a pure Ethereum mining rig you would essentially look to maximize raw performance per watt and performance per dollar invested. Cryptocurrency exchange platform? Then you can choose to keep coins or exchange it for other coins or currency.