Miele S6270 vs S2121
images from manufacturer

Some major difference between these 2 Miele canister vacuum serials are:

FilterBag:
The S6270 use 3.7 qt type F/J/M AirClean bag, while the S2121 use the 4.76 qt type G/N AirClean bag.

Silence Motor:
The S6270 features the new low-noise Silence Motor.

Cleaning Radius:
S6270 provides a 33 feet operating radius while the S2121 has a cleaning radius of 29.5 feet.

Parking System:
S6270: Rear parking system attaches floor tool to canister for breaks .
S2121: Dual parking positions – 2 parking positions for floor tool.

Floor Tool, Hose, Wands:
Both S6270 and S2121 have a few models, they may comes with different floor tools, hoses (non-electric suction hose or electric hose), and wands (telescopic stainless steel wand or electric telescopic wand).

Dimensions:
Dimensions of S6270 are: 9″ x 10″ x 17″ (H x W x L)
Dimensions of S2121 are: 8.5″ x 11.25″ x 18″ (H x W x L)

Weight:
S6270 weighs about 9.8 pounds while the S2121 weighs about 9.5 pounds.

Price:
Prices of S6270 models range from $399 to $749 at mielestore.com.
Prices of S2121 models range from $299 to $499.

They also may be in different colors.

If you know more difference between Miele S2121 and S6270, please share below.