There are some major difference between Shark NV80 and NV42 though they look similar.

image from manufacturer

If you want swivel feature, the NV80 fits your needs since it has the swivel steering which the NV42 does not have.
If you want a larger dirt bin, you should choose the NV80, it has 3X dust cup versus 2X on the NV42.
If you are a pet owner or allergy sufferer, the NV80 may be a better choice since it features Anti-Allergen Complete Seal Technology + HEPA filter.
If you want a cheap one with longer crevice tool, then buy the NV42 because it is about $20 cheaper and include a 8″ longer crevice tool.

More details, read the specs comparison table below.

Compare Shark NV42 and NV80 Specs

	Shark Navigator Deluxe NV42 Specs	Shark Navigator Professional NV80 Specs
Design	Upright	Upright
Bagged or Bagless	Bagless	Bagless
Clear Dirt Cup	Yes	Yes
Dirt Cup Size	Twice the capacity of the original Navigator	Three times that of the original Navigator
Power Source	Corded	Corded
Cord Length	25 foot	30 foot
Power	1200 Watts	1200 Watts
Amps	10 amps	10 amps
Nozzle Width	9.25"	9.5"
Weight	15.1 lbs	18.6 lbs
Never Loses Suction Technologys	Yes	Yes
Motorized Brushroll	Ye	Yes
Brushroll On/Off	Yes	Yes
Washable Filters	Yes	Yes
Headlights	No	No
HEPA Filtration	No	Yes
Filtration	Electrete Filters	Foam Filters and HEPA
Anti-Allergen Complete Seal Technology	No	Yes
Manual Height Adjustment	No	No
Bottom-empty dust cup	Yes	Yes
Attachments	a 5.5" Crevice Tool, 24" Crevice Tool, Dusting Brush, and a Pet Hair Power Brush	a 5.5" Crevice Tool, 16" Crevice Tool ,Dusting Brush, Pet Hair Power Brush
Handle	Fixed	Fixed
Cleaning Wand	1 wand without handle	2 wands, one with handle and one without handle
Hose	Yes, stretch hose	Yes, stretch hose
Speed options	Brush on or suction only	Brush on or suction only
Swivel Feature	No	Swivel steering for easy maneuver
Color	Gold	White with Gold
Self-propelled	No	No
Full Bin Indicator	No	No
Surface Cleaning Types	Carpet, Bare Floors	Carpet, Bare Floors
Warranty	5 year limited warranty	5 year limited warranty
Manual	Download Manual	Download Shark NV80 manual from Amazon: http://s3.amazonaws.com/szmanuals/dcdf1f182804a108681a9250f14573b3
Suction Control	There is a suction release valve at the top of the hose, you can adjust it to get the desired suction.	There is a suction release valve at the top of the hose, you can adjust it to get the desired suction.

Please comment below to share your reviews on Shark Navigator NV80/NV42 if you own these vacuums.

Shark NV80 versus NV70
For those who are searching for info about the difference between Shark NV70 and NV80, here is the answer from Shark customer support, they said that the NV70 and NV80 are the same unit just sold in different stores.
Yes, they look similar, they have similar specs and features such as Anti-Allergen Complete Seal Technology with HEPA filtration, 3X Dust Cup, Swivel Steering, etc.
But if you look closer, you will find that they are not identical, at least there are some tiny difference.
In the manual, you can find that the Shark NV80 comes with 2 Cleaning Wands, one with handle and an extra wand without handle, while the NV70 only has one without handle.

The included Pet Power Brushes are different. The one on the NV80 is wider.

The color scheme are different as you can see from the product images.

If you know other difference between these 2 Shark Navigator upright vacuum cleaners, please share your experience below.

Both of them equipped with a large dust collection cup, if you need a vacuum with large capacity, then these vacuum cleaners would be a good choice.

Here are some key difference between Shark NV42 and NV70 that you may want to know before buying.

image from manufacturer

Swivel Steering
The Shark NV70 features swivel steering that allows you to maneuver around obstacles easily while the NV42 does not have the swivel steering.

Dust Cup Capacity
The dust cup on the NV70 is larger, it is 3 times that of the original Navigator while the NV42 twice the capacity of the original Navigator.

Filtration
The Shark NV70 features Anti-Allergen Complete Seal Technology with HEPA Filter that is able to captures and hold up to 99.9% of dust and other allergens inside the vacuum, not back into the air you breathe, good for allergy sufferers and homes with pets, while the NV42 features Electrete Filters.

Included Tools
They both come with the Pet Hair Power Brush, however, the 2 power brushes are different.
According to the info on amazon.com product pages, the NV70 has a 16″ crevice tool while the NV42 has a longer crevice tool that is 24″.

Some similarities:
Power specs: 1200 Watts, 10 amps
Cleaning Path: 9.25″
Brush roller with brush On/Off settings for cleaning carpets and hard flooring
Clear Bottom Empty Dust Cup

Please share your experience on Shark NV42 or NV70 below if you own these Shark vacuum cleaners.

images from manufacturer

We will discuss the difference between Shark NV42 and NV22 (NV22L,NV22S…) below, the info may be helpful for those who are deciding between these Shark upright vacuum cleaners.
In fact, the Shark NV22, NV22L, NV22S, etc. are the same machine except for some small differences such as the color and included tools.

For the NV42 and NV22, according to Shark customer care, they are very similar units, mostly just some of the accessories.

The NV42 which is in Bisque color comes with a pet hair power brush, dusting brush, 5.5″ crevice tool, 24″ crevice tool.

The Shark NV22 includes one crevice tool(some packages include a 5.5″ one, some include a 24″ one) and a dusting brush.
The NV22L include the 5.5″ crevice tool, 24″ crevice tool(some packages do not include this one), dusting brush, and a pet hair power brush.
The NV22S include the 5.5″ crevice tool, 24″ crevice tool, dusting brush, and a pet hair power brush.
The NV22 which is sold only at walmart is Blue, the NV22L is the Lavender version, while the NV22S is Silver.

The NV42 has a cord length of 25 feet versus 30 feet on the NV22, NV22L, NV22S.

And it seems that the Extra Large Capacity Dust Cup on NV42 is larger than the NV22.

If you know other difference between the Shark NV22 series and NV42, please share your experience with others below.