The 300D is the W123 chassis and the 300SD is the larger W126 Chassis, and the early ones were thre W116 Chassis. Now the differeces are that the W123 is much smaller and have the two 7" headlights with round foglights. They are also much simpler and don't have power seats. The W126 is alot larger and has power seats and alot of other luxury features that the W123 does not have. Both however use similar turbo engines, except for the non-turbo 300D's. The 300D's are much simpler, so they can be less expensive to repair if you can do a lot of the work yourself. There is more that others can add to this.