Their canister unit are the same. Here are some difference between these 4 Miele S2 models:

Floor Tool:
S2121 Olympus: Classic Combination Carpet/Smooth (SBD 350-3)
S2121 Capri: Parquet (SBB Parquet-3), Turbo Comfort (STB 205-3)
S2121 Delphi: Electro Comfort (SEB 217-3)
S2181 Titan: Parquet (SBB Parquet-3), Electro Comfort (SEB 217-3)

Filter:
S2181 Titan include Active HEPA Filter while the other 3 vacuum cleaners include Air Clean Filter.
But they all support Active HEPA Filter.

Hose:
S2121 Olympus and S2121 Capri come with non-electric hose, while S2121 Delphi and S2181 Titan come with electric hose.

Deluxe Handle Grip with Electrobrush Control:
The S2121 Delphi and S2181 Titan have the deluxe handle grip with electrobrush control, while the S2121 Olympus and S2121 Capri do not feature.

Price:
S2181 Titan is the most expensive one while the S2121 Olympus is the cheapest one.
At http://mielestore.com on 2013-04-26,
S2121 Olympus is priced at $299
S2121 Capri is priced at $399
S2121 Delphi is priced at $499
S2181 Titan is priced at $599

Color:
S2121 Olympus is in Lotus White
S2121 Capri is in Lava Grey
S2121 Delphi is in Lotus White
S2181 Titan is in Mango Red