The Hohner Marine Band is a very good diatonic harmonica famous for its amazing tone. It is the instrument of choice for many professional harmonica players. The downside of a Marine Band is that it has a pearwood comb which tends to swell up from moisture.
The Hohner Special 20 is essentially a marine band with a plastic comb. Plastic is resistant to moisture, and this makes the Special 20 an easier to maintain harmonica. Another good thing about the special 20 is that it is put together with screws, not nails (like the marine band). So, it's easier to open a Special 20 for repairs or cleaning.