If you think about service professionals, and by service professionals I mean anyone that does something that doesn't create a product, it is easy to see why it would be difficult to tell whether one is better than the other. They all look kind of the same, they all say the same thing, and at the end of the day, you're not really sure whether or not someone could have done what you had done for cheaper, or even gotten a better result.
This feeling holds true for criminal attorneys too. I mean, I'm a criminal attorney in Seattle, I have my own practice, and I am constantly thinking about ways that I can differentiate myself from other criminal lawyers in the area so that people will feel like they are hiring me specifically for a very specific reason.
You try to think about it for a second. If you were an attorney, a doctor, a dentist, a plumber, a mechanic, a wealth manager, or a photographer, how would you show people that you are different. And notice that I said show, because you can't just talk about how you are different. Everyone does that. Start looking for criminal lawyers and you'll see buzzwords like "experienced" and "aggressive" but nothing that really shows that.
Think about what you could do as a mechanic to have someone say "I hired this person for X reason," and really have it differentiate you as a service provider. What you'll get most of the time was "I thought they were okay and their rates were reasonable so I picked them." That isn't the type of feeling that has people banging down your doors.
To answer the question, though, yes, there is a difference between one criminal lawyer and another. For example, I said I was a Seattle criminal attorney. I know many many criminal attorneys out there. Some are definitely better than others. But the only reason I know that is because I see them in action. As a consumer, you don't get to have that experience beforehand. You have to guess.
So what's the trick? First, don't pick the cheapest guy, ever. You are getting what you pay for. Second, find someone that you like. Third, find someone that other people like (references or referrals are always good signs). And finally, talk to several people before making a decision so you can see the differences in personality.