In this post, DC15 refers to DC15 Animal, DC25 refers to DC25 Animal.
Dyson DC15 VS DC25 Comparison
images from manufacturer

Compared to DC15, the big cons of DC25: smaller dirt container and shorter cord. But the DC25 is lighter.

Bin Capacity
DC15 has a much larger bin.
The bin capacity of DC15 is 0.71 gallons while DC25 has a 0.31 gallon bin.

Cord Length
DC15 have a much longer cord.
The cord length of DC15 is 37 ft cord whereas DC25 has a 24.6 ft cord.

Motor Rating
DC15 has a 10 amp motor while DC25 has a 11 amp motor.

Size
The dimensions of DC15 are 44.09″ x 13.66″ x 11.41″ while the dimensions of DC25 are 42.4 x 12.2 x 15.4 (HxWxD).

Weight
DC25 is lighter.
DC15 weighs 19.95 lbs while DC25 weighs only 16.12 lbs.

Tools
DC15 comes with a carpet care kit, mini turbine head and low-reach floor tool; whereas DC25 comes with a combination accessory tool, mini turbine head, and stair tool.

Price
DC15 is cheaper.
At some online stores, DC15 costs less than $400 while DC25 costs about $500 when this article posted.

Both of the vacuum cleaners are bagless and have suction power of 220 airwatts; they feature Dyson’s Ball technology, Root Cyclone technology; come with motorized brushbar, lifetime washable HEPA filters, telescope reach wand, and carry 5 years warranty.