It's actually only 20%, but they'll still pay you more than most of your existing agencies at the end of the month. This is of course an assumption, but one based on the shared experiences of other microstockers large and small.

iStockphoto is extra work compared to the others, but they make up for it with high sales volume. It's unlikely you'll regret it.

PS. there's no such this as a "correct choice". 

I suggest IStock and StockXpert

If you have IPTC data on all your images you can FTP to LO and click submit with minimum effort

Similarly you can burn a DVD and post it to SV they will add it for you.

IS is definitely worth it - upload in order of your top sales, the least effort for the most return
their non exclusive commission may be low at 20% but their sales are higher than most other sites so it is well worth it.

I would probably point you to StockXpert.

I have a very small folio there but decent enough earnings for its size.
After that FP, but sales are very intermittent there.

Getty and Corbis

...or Alamy and Photoshelter for Rights Managed travel shots of a less generic nature.  The sort you might take on holiday or on a Sunday outing.