I love the sebenzas. I have 5 and ALL are regulars. I have 3 large regular and 2 small regulars. I am sure someday I will have the classic/21 style. IMHO, I like the blade profile on the regular verses the 21 style. Its more of a drop point where as the 21 is more pointy. Both will do what you need it to do, so that part of it just comes down to what blade shape you like. I like the large opening around the thumb stud on the classic style, it looks like it would be much easier to open. But I like the grooves on that are on the regular as it gives you a better grip. From what I have read, the only difference between the classic and the 21 is the 21 is more beveled on the edges to slide in your pocket better.
As for large or small, thats the great debate. I love the large regular, but when I pull it out to use, people notice. Its not overly large by any means. I thought it was actually smaller than I had imagined. When I pull the small out, no one notices. I use and carry a small regular vines sebenza for edc, but I would prefer to carry the large. I bought a large regular for edc and am shipping it off monday to CRK to have it custom engraved. ( I will post pics when I get it back ). I have no complaints with my small vines edc sebbie, it does everything I ask of it. So IMHO, The difference between the regular or classic is just basically what appeals to you. In the end, you will never go wrong with either one. Both are knives of the Highest Quality and are Extremely Durable. Plus, you have the best Customer Service in the industry and a lifetime guarantee.