The biggest differences are time and resources.
A public defender is employed by the state. They are required to take every case assigned to them. What does that mean? They have a HUGE caseload. They might be assigned to upwards of 200 cases at a time. And the more cases they have, the less time they are going to spend on yours. They might miss a critical element just because they’re so bogged down with other cases. Also—a public defender gets paid the same amount no matter how many cases they get stuck with, or how many cases they win/lose. They don’t have the same incentive to help you. They don’t have a business based on referrals.
You want someone who has a long-term interest in your success, and unfortunately, public defenders don’t.
Public defenders also have fewer resources. Their means of helping you are limited by what the state gives them access to. They don’t have an array of superior resources available such as private investigators, the way a private attorney does.
The best legal defenses take time and research. That’s why you are better off going with a private criminal attorney –one who can devote as much time as is necessary to win your case, and who has the resources to get the job done right.