No No there is a very basic difference between both of them. Coinbase is a centralized wallet that does not give you private key of your bitcoins while Mycelium is just a wallet holder which gives you complete freedom and control over your wallet. You cannot buy or sell inside a private key wallet you have to move your money to an exchange to sell it and that would require transaction fees in Mycelium while Coinbase does not store your money in a specific private key they just do internal shuffling of your bitcoin whenever you put a buy or sell order which is why all of that is free. If you want to get the real feel of bitcoin and it's anonymity Mycelium is the thing for you but if you just want another go to exchange where you can buy/ sell as per your ease then go for Coinbase.