When it comes to picking your altcoins, do you take into consideration coin supply? And if so, do you prefer high or low supply coins?
No, I think it's irrelevant. Low supply means high individual coin price, high supply means low individual coin price. Supply is only relevant when linked with coin price... i.e., market cap.
The only relevance I can see for supply is current circulating supply versus total potential supply. The lower the percentage that is currently in circulation, the more chance there is of price dilution, i.e. reduction.