In general: One may treat long and short the same. Some ETFs are inverse. But, for stock markets, and depending on the market, long and short side are statistically different. Short tends to be fast, brief and high vol, while long tends to drift slowly upwards on lower vol. For any extreme case, there are many cases in between also, that tend to not work the same. Thus one need to learn how to "read the market", and be adaptive to what's happening when.
There are too many factors involved to give a meaningful direct answer to your query. But, all things being equal, and generally speaking: odds favor the long side. Thought experiment: Consider a typical and random chart. Imagine picking two random dates on that chart. The first date being entry and the second being the exit. Repeat this simulation many thousand times--considering a new random chart each time. Ask and answer for yourself: What are the odds that the exit price is higher than the entry price? That's the bias.
Shorting stock indices during the right circumstances is comparatively easy, it's just that the right circumstances for shorting them are very infrequent and it takes patience to wait for them (during which you're better off being long, unless in a pronounced crisis market like 2008).
I can only speak for myself, but moves to the downside usually seem more clear to me than the ones to the upside.
True but timing shorts is a lot harder since markets go up longer-term. This is an interesting article with statistics about SP500 from the same expert I mentioned in another post.
In the book Long Term Day Traders, a number of very successful traders are interviewed by the authors: Linda Bradford Raschke (the only woman to make Jack Schwager's famous Market Wizard book) - is reported as making 70% of her money from the short side. Oliver Velez (Pristine Capital) - says "The short side is very lucrative. Really true master traders have the majority of their profits come from the short side, because stocks have a tendency to fall faster than they rise."