We will discuss the difference between Dyson DC65 and DC33 below.
Both of them are full size upright vacuum cleaners, the DC65 is a newer model that has some features that are quite different from the DC33.

Movement Type
One of the major differences is the movement types they employ. The DC65 uses Dyson Ball whereas the DC33 uses fixed wheels.
Dyson claims the Ball provides better maneuverability, allows you to get it around easier. However, it is hard to say which type is better, it really comes down to personal preference.

Cyclone Technology
DC33 features Root Cyclone technology whereas the DC65 employs the Radial Root Cyclone technology that remodels the airflow to provides greater suction power.

Cleaner head
The DC65 cleaner head is a bit lower, Dyson claims that there is a redesign on the brushbar that makes the DC65 is more powerful for cleaning carpet.

Using Wand with Hose
For the DC33, it allows you to hold the handle while using wand and hose. Whereas the DC65 has no handle to hold.
Here are images from Dyson product instroduction, you can see the difference from the images.

Bin Capacity
The DC33 dust cup capacity is a bit larger.

Brushbar Control Placement
The brush control button on DC65 is next to the Power switch that is located above the charmber part, this allows you to turn on/off the brushbar without bending down.
Whereas the D33 has the brushbar on/off switch located at the bottom that is not as convenient as a fingertip control.

Removing Blockages from the Brushbar
It is easier to remove blockage from the DC65 brushbar.
For DC65, you can detach the cleaner head from the unit to remove blockage like long pet hair, you can also remove the brush bar by taking off the screws.
For DC33, the brushbar is not removable, you will have to sit on the floor to clear the blockage.

Compare Dyson DC65 and DC33 Side by Side

	Dyson DC33 Multi Floor Specs	Dyson DC65 Animal Complete Specs
Images
Cyclone Technology Type	Root Cyclone™ technology	Radial Root Cyclone™ technology
Movement Type	Fixed Wheel	Ball Technology
Design	Full Size Upright	Full Size Upright
Multiple Floor Types Cleaning	YES	YES
Suction Power	240 airwatts	245 airwatts
Suction Control	No	No
Bagless	YES	YES
Bin Capacity	0.61 gallons	0.55 gallons
Corded	YES	YES
Cord Winding	manual	manual
Cord Length	35 ft	35 ft
Max Reach	51.1 ft	51.7 ft
Wand	Instant Release	Instant high reach wand
Cleaner Head Width	about 13.4″	about 13.3″
Weight	17.6 lbs	17.35 lbs
Motorized Brushroll	Yes	Yes
Brushbar On/Off Location	at the vacuum head	above cyclone part
Manual Height Adjustment	No	No
HEPA filtration	YES	YES
Warranty	5 year parts and labor warranty	5 year warranty on parts & labor
Dimensions	44.9" x 13.4" x 14.0"	42.4" x 13.4" x 15.4"
User Guide	DC33 Multi Floor User Guide	DC65 User Guide
Tools included	Combination tool Stair tool	Tangle-free Turbine tool Combination crevice / stair tool Stiff bristle brush Soft dusting brush Multi-angle brush Zorb carpet maintenance powder

According to user reviews, the noise level of DC65 is lower and it picks up pet hair from area rug better.
The pre motor and post motor filters and filter location have been changed, but not a big deal.

If you own Dyson DC33 or DC65, please add more info about the comparison, thanks.

The Dyson DC65 has a similar size to DC25, but has stronger suction power.
Dyson claim that the DC65 has a reconfigured brush bar that generates 25% more power drives bristles deeper into carpets removing more dirt, and the Radial Root Cyclone technology remodeled air flows help remove more dirt, dust and allergens.
Both of them include a few models. For example, the DC65 includes DC65 Animal, DC65 Animal Complete, and DC65 Multi Floor, these models are the same unit except for the color and included tools.

Here are some differences between Dyson DC25 and DC65 that you may want to know before making a purchase.

Bin Capacity
The Dyson DC65 has a much larger dust cup.

Cord Length
The cord on the DC65 is over 10 feet longer than the one on the DC25.

Cleaning Path
The DC65 has a wider cleaner head.

Weight
The DC25 is about 1 pound lighter than the DC65.

Using Wand with Hose
When using the wand with the hose to clean around, the DC65 has NO handle on the wand while the DC25 has the handle on the wand that puts less stress on your wrist.
Here are pictures taken from the manuals that will illustrate it clearly.

Dyson DC25 and DC65 Side by Side Comparison

	Dyson DC25 Animal	Dyson DC65 Animal
Images
Cyclone Technology Type	Root Cyclone™ technology	Radial Root Cyclone™ technology
Ball Technology	YES	YES
Design	Full Size Upright	Full Size Upright
Multiple Floor Types Cleaning	YES	YES
Suction Power	220 airwatts	245 airwatts
Amps	11 amps	11.7 amps
Bagless	YES	YES
Bin Capacity	0.31 gallons	0.55 gallons
Corded	YES	YES
Cord Length	24.6 ft	35 ft
Auto Cord Rewind	No	No
Max Reach	40.3 ft	51.7 ft
Cleaner Head Width	about 12″	about 13.3″
Cleaner Head Technology	Self-adjusting	Self-adjusting
Warranty	5 year warranty on parts & labor	5 year parts and labor warranty
Dimensions	42.4" x 12.2" x 15.4"	42.4" x 13.4" x 15.4"
Weight	16.125 lbs	17.35 lbs
Motorized Brushroll	YES	YES
Brush On/Off	YES	YES
Suction Control	No	No
Manual Height Settings for Carpets	No	No
HEPA filter	YES	YES
Manual	Dyson DC25 Manual Official Download Link	Dyson DC65 Guide

If you own Dyson DC65 or DC25, please share your experience, reviews, and more info below.