Alamy is not at all exacting beyond meeting technical requirements (noise, focus, size minimums) - they don't edit the collection at all. SS will reject in a heartbeat for LCV (low commercial value) and composition. Do try to select your submission for SS very carefully, and don't just assume they'll take anything Alamy will.
iStock has a very different take on things from SS. SS likes punchy and attention grabbing; iStock prefers technical perfection even if it's a bit boring. As far as applying to iStock, look at the earnings chart on the right - they're #2 on the list. Why wouldn't you want them selling your images? They are owned by Getty and Getty behaves very badly to photographers, musicians (contributors), but they sell a lot of work.
