the Marine Band 1896 and Special 20 are both great harmonicas and both are fitted with the same Hohner Classic reeds, the most important factor in defining the sound. As Harmonicat has pointed out, the difference lies in the construction - the MB employs the traditional sandwich design, where your lips touch the edges of the brass reed plates, whereas the Sp. 20 features recessed reed plates and a plastic mouthpiece. Some people find this more comfortable. Both models have stainless steel covers, nickel plating was phased out a long time ago. The Special 20 is a true workhorse instrument and one of the most copied harmonica designs ever.
Personally I have a clear preference for the Marine Band, though I’m anything but a purist. It has an inimitable sound which is the reason for its lasting popularity and has made it the benchmark for diatonic harmonicas. In order to make it more comfortable to play and to improve ease of servicing, we developed the Marine Band Deluxe and Crossover models, both of which are fully assembled with screws and feature improved cover and comb design. I play both of these interchangeably. If you like the MB sound, you may want to try one of them.
However, if the Special 20 strikes a chord with you, then as Harmonicat suggests, you may want to try a Rocket. Here we introduced a number of new features including an ergonomic design where all outer edges are rounded for greater playing comfort, as well as wider channels and improved cover design for greater volume.
In the end, with any musical instrument it’s all down to personal preference. The harmonicas of both Marine Band and Progressive Series are all top quality instruments and share many of the same tonal characteristics due to the excellent reeds, so really it’s a matter of what you feel most comfortable with.
However you decide to choose, I hope you enjoy playing them,