Each one will have it's advantages. A lot of those will come down to you.
Some affiliates want everything done for them. While others will generate things themselves to be original. The smaller networks may not be as saturated, but may also not have a ton of support. While bigger will have more ad tools and support, but also more affiliates, and possible saturation.
It really comes down to how good you are as a marketer in the end.
:rasta: