Availability, I prefer going with smaller agencies that will work their tail off to keep you as a customer rather than becoming a second-class project for bigger agencies that will outsource the contract to said smaller companies anyway.

I think finding a young team of hungry SEOers is better than sticking with older, more established agencies.

But it all depends on what you’re looking for…

A) What’s your company’s business?

B) Are you a small or medium company?

C) How much are you ready to invest in SEO?

A smaller but consistent engagement is better than big expensive pushes every few months.

I hope this can help you out!

I am objective, and do not work for any web agency. I just worked with a lot of them in the last 6-7 years.