For me, in pure numbers..   Highest volume of pix sold,  SS.  Fastest money-maker, IS.  Most EL's,  SS.
    As some have already said here,  they all work differently for everyone.    I don't think there's any cut & dry fromula on which is the best.  Example, just reading on another thread.  Topic,  BigStock vs 123rf.  Some love 123rf, for others it's a low earner,  and vice-versa.  For me, BigStock is my #4 earner and 123 is 8th out of 9.   
     You'll find out what's good for you in time.  ...wishing you success, welcome to the club!!   -tom

  Do you know if you can still get accepted if only one sample passes inspection?

The procedure is somewhat different at IS. They simply give yopu a verdict, you are in or you are out. There may be slight variations like "we loved two photos, third is questionable, send us one to convince us". But those three candidates are not gouing in your portfolio right away when you are accepted vs how it's done at SS. You will have to submit them again for normal reviwe process, and they do not necessary get approved.

ALMOOOOST!!!!   

I got an email from iStock, turns out one of my photos passed inspection, but the other two failed.  This confirms the need to have all three pass inspection in order to get accepted.

The good news though is that I called it perfectly.  I only felt one of the three photos would pass, but I was hoping one would be enough.  The one photo accepted and the two rejected were based on the very same conclusions I made myself, which shows I'm thinking along iStock's wavelengths in terms pf photo quality, so that's encouraging.

The other good news is that I can use samples that normally would not be accepted into iStock's inventory.  I didn't know that, and because of it I was pretty limited in choosing what samples I could upload.  Most of my photos are landmark or landscape photos, generally the kind that iStock either already have an abundance of or require a property release for.  But for the application process they're just looking to evaluate the quality of my photos.  Sheesh, I wish I had figured that out the first time around.

They got back to me pretty fast though, and suggested I replace the two failed samples with fresh new ones for a second review.

If at first you don't succeed.....   

Yep, although some people here have success with BigStock, my own experience is that it is a complete waste of time.

Lincoln, please change your camera and get a DSLR.  You could find a used Canon 350D or 400D on eBay for not much money.  You can then concentrate on creating great pictures instead of spending so much time wondering how to achieve what you want with your existing camera.