Before you choose a data recovery company, you should find
out if they are a company that is dedicated only to data recovery.  Data recovery is a complicated and complex process.  Therefore,
make sure you are dealing with a company who specializes in recovery and not primarily computer repair. 

A good recovery company also has the capacity to work on any
operating system. Don't make a mistake by sending your Mac
drive to a recovery company who only specializes in Windows
data recovery. 

Many data recovery companies offer emergency data recovery
service which allows your job to be placed higher in the recovery
queue which can reduce downtime. 

Another factor to be considered is upfront costs.  Some recovery companies offer a free estimate while others charge a flat fee to
evaluate your media- even if the recovery is not successful.  Also consider how the data recovery company calculates their final
price for the recovery and make sure you get an idea of this price upfront.  Data recovery can be very expensive.  It is a good idea
to get the cost upfront instead of being shocked at the final price
in the end.  

Also keep in mind that recovery companies offering prices that
seem considerably lower than the norm often lack necessary experience to perform invasive hardware recovery requiring parts.
Remember, in most cases you only get one shot to recover
data.

By using vigilance and the tips above, you can hopefully be on
the road to a successful recovery of your data.