On the other hand, the one file I have that sells well on the PP has sold a grand total of 10 times on IS since 2007, garnering a whole $15.16, whereas on the PP it has sold 329 times getting $135.26, although there is nothing special about it and there are similars.

You just can't tell.  

Isn't that the real difficulty in this business. We can't work out anything based on our own past experience, or that of others.

As a kind person pointed out, my presence on IS is too small to draw conclusions but, bugger it, I'll comment anyway..

I don't see much of a difference between PP and subs sales anywhere else except the return per download is a bt lower but the return per image ain't bad as relative volumes are very high;
Different files seem to sell better there vs IS itself and vica versa;
Never had an 8 or 9 cent download but have had a few for around $4 and even a $22 sale (higher than I ever had on IS itself).
However bad it might be it's gotta provide a better return with no effort than putting stuff on mid and low tier sites.

I agree with JoAnn.  I am absolutely certain that TS has been largely responsible for the drastic drops in download numbers at Istock.  This has only increased as prices for exclusive images have risen at IS.  Buyers are being actively redirected to TS from Istock.

That said, as an independent, my images are there whether I like it or not.  In light of Istock's plummeting sales, the only thing keeping my istock income afloat is the PP income being added to the totals.  It is not an even trade though.  The hundreds I've gotten from TS doesn't begin to replace the thousands I have lost on Istock. 

Agreeing with you but to a point. IS, have over the last couple of years dragged itself in the mudd and reputation in this business is vital, not only with buyers but also contributors, new, and I mean professional photographers are scared to join up with IS, simply because of this everlasting "war" between exclusivity and independancy, RC, cuts and whatnots. Buyers do not want to get mixed up in all this sh#t and turn to others.
However ofcourse TS, has had a major influence on IS, when buyers are getting re-directed to TS, its obvious.

On a personal stand, I have to say, I actually do prefer Getty/TS, then the weak, nerdy present IS-admin, who seems to get constipated every 5 minutes, spending more time in the bog then in the office.