if in a long situation which would be better - short the QID or go long the QLD?
They are supposed to be mirror images but they are not -
They are close but the problem right now is that there just isn't enough data or experience with these two funds but right now the fact is the QLD has "longer legs" than the QID.
That would suggest that anytime you have an opportunity to buy QLD vs short QID you probably should and vice versa - I.E. if the occasion suggests being long QID you probably should short QLD. As an interesting aside - the actual short interest on QID is nearly 3 times that of QLD - so we need to go to the raw figures to work it out.
Putting aside expenses and fees associated with ETFs and shorting which will change the results a bit this is what I found using 5 different "long market" periods in the past 150 days -
QLD per share total returned 13.50 and short QID per share total returned 10.03.This bears out the visual above (QLD having "longer legs") and suggests that people who short these two stocks really don't know what they are doing because they are shorting the wrong one.
Again - the sample size is too small to say with any conviction do this or do that. All I can say is that up till now it appears a long purchase of QLD when the market goes up and a short sale of QLD when the market goes down might be the appropriate way to go. A year from now the story may change significantly.
I hope that answers the question and I'm always glad to help out.