Chris
Grab a bottle of wine and spend the evening reading through forum archives. This will give you a better, more eloquent education than i can type here.
Essentially there are originals like "Bessie" and everything else is a modern variant of that original inspiration. All makes of seven are great: regardless of whether you have a scratch built locost that sheds fenders, or a storker that sheds the earth we all love our cars.
They aren't for everyone though. So look around, ask lots of questions, sit in a few as proper fit is key. If you're still interested come to the annual gathering of sevens in July.
p.