S5211 Ariel vs S5281 Callisto
images from manufacturer

The difference between Miele S5281 Callisto and S5211 Ariel are:

Included Filter:
S5281 comes with Active HEPA Filter
S5211 comes with Air Clean Filter
The HEPA filter does a better job capturing tiny particles.

Floor Tools:
The S5281 comes with SBB300-3 Parquet Twister and Electro Plus (SEB 228) ElectroPlus floor tools.
The SBB300-3 Parquet Twister rotates 180-degrees and is agile when cleaning around obstacles such as chair legs.
Miele claim that the SEB 228 ElectroPlus electrically-driven power head is for vacuuming medium to high pile and plush carpets.
The S5211 comes with the SBD285 Combination carpet/smooth floor tool that is for cleaning bare floors and low-pile carpeting. With the foot switch, you can easily switch between bare floors and carpets.

Wands:
S5281: SET220 Electric, Stainless steel telescopic wand
S5211: Stainless steel telescopic wand

Hose:
S5281: SES119 electric hose
S5211: non-electric hose

Color:
S5281 is in Deep Black while the S5211 is in Polar Ice.

Price:
The S5281 is $350+ more expensive than the S5211 (prices checked on 05-14-2013).

If you know more difference between S5281 and S5211 S5 models, please share your comparison below.