We often have beginning banjo students ask, "What's the difference between a flathead banjo and an archtop? Do I need an archtop if I want to sound like Ralph Stanley, and a flathead if I want to sound like Earl?" When they ask this, they're referring to the kind of tone-ring the banjo has. That's the big metal part that sits right under the plastic banjo head, on top of the banjo's wooden shell.
Well, the truth is that in one way, it isn't a simple question to answer. The best of the old Gibson flathead banjos had a characteristic powerful, low-end resonance that Earl took advantage of when he played, and which helped make his much-admired sound. But we have to remember that it was Earl playing, and he'd have sounded like himself whether the banjo had a flathead tone ring, an archtop one, or no tone ring at all (as when he was playing with Bill Monroe in 1945-7 and used a banjo with just a little tone hoop). In all those situations, he still sounded and sounds like Earl.
Some folks like to have an archtop banjo so they can "sound more like Ralph Stanley." Frankly, it is fun to play 'Little Maggie' and 'Clinch Mountain Backstep' and hear that higher timbre come out of the banjo. But you don't need an archtop tone ring to make it that way, because (1) banjos like Murphy's Stelling have plenty of high end to go with the low end flathead sound, and (2) you can adjust your hand position on any banjo to get more of that high end out of it.
That sounds complicated. What's the answer? Well, it's simple. If you want to play a particular kind of banjo music, LISTEN to it and PLAY THAT SOUND. It doesn't matter what design of banjo you have, as long as it's a decent-quality instrument. The better the banjo the better you'll sound, generally speaking, but you can certainly play Earl's music on an archtop banjo, as Little Roy Lewis did for years, and you can certainly play Ralph's music on a flathead as some pickers (like myself) have done for a long time. So what makes the difference? What makes the difference is YOU. You need to LISTEN over and over to the music you want to play, and play not just the notes, but the SOUND.