BISSELL 66T6 vs 66T61
images from manufacturer

Here is similarities and difference between BISSELL 66T61 and 66T6:

Difference:

Floor Nozzle:
The 66T6’s floor nozzle has a Pet Hair Lifter to attract hair while you vacuum so you can pick up more pet hair than with suction alone.
The 66T61 does not have this feature.

Tools:
The 66T6 comes with 2 pet hair tools: The Pet Contour Tool and the Wide Pet TurboBrush Tool, while the 66T61 does not have these attachments.
Users can use the Pet Contour Tool to pull hair from hard-to-reach areas, and use the Wide Pet TurboBrush Tool which has a rotating brush and rubber fingers to pick up pet hair from furniture and stairs.

Color:
66T6 is black pearl while 66T61 is blue.

Price: 66T61 is cheaper.
66T6 is priced at $129.99 while the 66T61 is priced at $99.99 at bissell.com (prices checked on 04-18-2013).

Similarities:
Motorized Brush: No
Attachments: Crevice Tool, Dusting Brush, Upholstery Tool
Telescoping Wand: Yes
Warranty: 1 year limited warranty
Bagged or Bagless: Bagless
Power Rating: 10 amps
Clean Filter Indicator: Yes
Variable Suction Control: Yes
Foot Width: 10.6″
Cord Length: 17 Feet
Automatic Cord Rewind: Yes
Clean Filter Indicator: Yes
Dual HEPA Media Filters: Yes
Hose Length: 6’9″
Cyclonic Cleaning System: Yes
Air Flow Regulator: Yes
Design: Canister
Weight: 15 Pounds