It depends on timeframe and how we define bullmarket. We are likely closer to a big bear move. I think it´s best to try to master your thing. Few become the best in several fields i guess. Did not hold position overnight.
I wouldnt really describe going short vs going long as different fields, but each to their own i guess.
The markets are up over 50% since March 2009. Only taking shorts would have drastically decreased your chances of keeping your money, as opposed to only taking longs, no matter how much you think you may have "mastered" the "field".
If you hold overnight, you can say that now, in retrospect; but seriously, when was the last time we woke up to a "white swan event" where the market gapped up 500 points or more? The chances of a news-driven disaster move down is far greater than a move of similar magnitude to the upside. So if you have a short bias, wait for a good short setup and hold short positions overnight, I believe the overall risk is less than holding longs. Intraday it doesn't matter, price trends up and down and if you're not fighting a strong intraday uptrend, you can make plenty to the short side nearly every day.
By definition of an oscillating uptrend, the price moves up more than it moves down. Obviously by definition there was more up movement than there was down movement since March 09. And within those up days overall, the dips had to have been smaller on average than the rallies. This means that shorting opportunities on average are less frequent and rewarding than long opportunities in a bull market. This is why you want to buy dips in a uptrend. The opposite is true for a bear market. Just because it's in retrospect doesn't mean it isn't true or useful in the future. My point is that being strictly committed to either shorts or longs, regardless of the trend, is a clear disadvantage if you want to trade any time frame. Perma-bulls overall had this problem in the last bear market, and perma-bears are having this problem overall in the bull market of the past year. The advantaged trader would change bias as the market environment changes. This has nothing to do with black or white swan events.