It's a tough one to answer without a crystal ball

Big:  More marketing power, therefore a bigger audience/more buyers... but more competition from a higher number of artists.

Small:  Fewer artists to compete with, but less marketing power, smaller audience/number of buyers.

The biggest seller for most people is Shutterstock.  I hung around in smaller agencies for quite a while, getting excited about the odd sale here and there, before finding the courage to go for the 'big two' - and I was really stunned as something like 80 sales ticked through in the first day on Shutterstock when I started launching my port there.  At that time my port had only about 150 images in it, so it was a great return.  I remember pressing 'F5' and just repeatedly dropping my jaw throughout the day.  Ahhhh... happy memory  

ETA:  You seem to be mentioning Photaki.com an awful lot in your posts. 

Stockfresh is just starting up (but the owners ran the very successful StockXpert before it was bought by Getty).  They're offering 50%, and many of us expect they will do well, once they've fully launched.

However, you may have noticed that some contributors are disgruntled over the length of time it's taking to get their initial review.  Once you're in, reviews are within 24 hours.

Thanks for that info Leaf!! It�s deffinitely a very interesting thing to think about- The whole smaller community feel does appeal to me alot! The small will never get bigger or improve their earning potential if we all shun them!