And selling an altcoin to bitcoin is much better but it will depends on how much you have bought in. So it will be much profitable and gain more satoshi.
Yes it's always a bad idea to trade bitcoin for altcoin as altcoin's value depends on bitcoin and not the vice versa. Again it depends from a person to person even I did trade bitcoin for litecoin in the past and that trade was fruitful to me on a short term. We could rather directly purchase altcoin instead of selling bitcoin in order to buy altcoin, as the value of bitcoin will increase for sure if altcoin has to rise and moreover if we used fiat to buy tokens/altcoin (only potential) then it will be more profitable.