Mechanically it's always easier to buy stocks (aka go long). Short-selling involves borrowing stocks and sometimes there are no available stocks to be borrowed. But as Yogi pointed out, you have to trade in the direction of the extant trend and if that direction is down, you have to go short or not trade that particular stock. Your greatest challenge will be getting the trend direction right, not going long or going short.
EDIT: Fyi, if you trade forex instead of stocks, it's mechanically just as easy to go short as to go long. And the trading hours are more convenient.