There are a number of comparisons out there. The biggest difference overall IMO is where the data comes from. KVS is generally screen-scraping publicly available websites and rendering the data in a different view. EF has direct access to various GDS feeds and gets the data that way. Depending on what you bits of functionality you want and what airlines you are interested in each has its advantages.

What are the search parameters you're using. Among other things, AA won't work because they don't publish their data into the underlying data source (FlightStats). Other than that it should pretty much work. But if you're having trouble I can definitely look into it. Feel free to PM me details if you don't want to share them here.