For me I sell all the Altcoins that I mine basically for bitcoins because it's gaining value than all cryptocurrencies and it's also part of my dream to raise at least a total 21 BTC and save for the future.
If you look at the alt coin market all the coins are having a good rally and if you are mining and were holding some coins then the chances of making a 100% return is very much possible with the kind of rally every coin had in recent months.
Mining altcoins is a better choice at a moment then mining bitcoin as mining bitcoin will require a good amount of capital to invest and nowadays its not that easy to mine bitcoins as it was in the past and if you don't have higher capital to invest in mining then better choice would be to start trading with some descent capital to make regular profits.