If you are mining coins then it does not really matter if you sell them right away or save them for the future. Either way you will make a profit so it is really up to you. If you want to make more money then waiting for the price to go up is the option to choose, if not then sell them right away.
I mine the bitcoin. I keep most of them. I just use some to cover my cost. I will make more profit if I keep them longer.
Unit value of mined coins will be same but exchange rate of certain mined coin may be vary. It depends up on market condition so its better to sell mined coins and invest in newly growing altcoins as price will up then sell them and get profit residually.
That's actually not a bad idea, but I think it would require a lot of research and so into the different coins and it simply might not be worth it for a lot of miners to take their additional mining income and use it to trade altcoins. There is a very selective number of alts that could actually maintain or procure some value, and those alts typically don't last for long.