What is your budget and when are you travelling?
Are you just looking for a stop on your way to The Keys, or are you going to spend time in Miami?
If spending time in Miami, I would look for a much more central hotel either downtown or on Miami Beach. It will add some drivetime, but you will enjoy your stay in Miami much more.
If just looking for a stopping point on the way to The Keys, there are some affordable hotels in Kendall or Homestead. Key Largo is only about an hour past where 41 meets the Florida Turnpike in Miami. Have you considered just continuing on to Key Largo and getting your Keys trip started a day early?