None of these things are easy comparisons for all the reasons noted above. In my case, I have a few from my iStock top 20 that are also in my Shutterstock top 20. In a couple of cases the download numbers are about comparable (two were 500-ish on istock) but the money total is much better for iStock.
But that isn't the whole story - iStock is (IMO) on a downward trajectory and SS upward, so although it would have been nice if iStock had continued as they were when those files were earning so well, that wasn't happening, which was a big reason for me leaving exclusivity. 6 of my SS top 20 are images created since I left iStock, so I don't have anything comparable.
I no longer upload to iStock, but as an indie, the returns are now so low that SS would be beating it easily.
The other thing looking at SS sellers, is that I have a couple in the top 20 that were there from my earlier stint as an indie and the lower $$ totals for a given number of downloads is striking compared to newer files (that have benefitted from the shift away from subscriptions at SS).