A technical comparison between the models is here:

https://www.ironkey.com/compare-hardware

More detailed information which you might find useful is as follows:

D200 is suitable for office workers who use their drive for file transfers of large amounts of data to share with others, bring work home, or perform periodic backups.

S200 is for Power Users who place heavy daily backup demands on their drives, use the drive for portability of a virtual desktop, or regular use of the on-board applications such as the secure browser.

The difference in more detail is the the D200 uses Mulitple Level Cell (MLC) memory and the S200 uses Single Level Cell (SLC) memory. This gives them different speed and lifespan attributes.

The speed ratios between them vary with the size of the drive but to generalize, the SLC memory drive is faster by about 20% at the larger size drives. The difference is greater in the smaller size drives.

SLC memory also lasts longer than MLC memory as measured by the number of write operations on the drive. While MLC memory is adequate for most casual uses, the SLC lifespan is estimated at 7 to 10 times longer than MLC and is recommended for uses and applications related to business continuity.

It’s sort of like buying a cheap computer instead of a more expensive one. You pay more for the more expensive computer, but that more expensive computer is most likely faster, and/or has more features.