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Shark S3101 and S3501 are similar simple steamers. They work in the same way. While using them, you pump the handle to produce steam. However, there are differences between them.
1, Mop Head and Pads
The S3101 is smaller to get into smaller areas.
Their heads and pads are completely different.
The S3501 has a swivel head, the pads for S3501 fit over the rectangular mop head and provide double-sided cleaning. So after one side of the pad gets dirty, you can just flip over the head to use the clean side of the pad without switching pads. Whereas the S3101 does not have swivel steering, the pad only provides one side cleaning.
The S3501 mop head and mop body are separate parts. You can easily exchange mop heads by pressing the release button to detach the mop head from the mop body, and snap other heads.
Whereas the S3101 seems to have the head built into the mop body. So it does not allow you to use an alternative head such as an triangle mop head.

2, Water Capacity
The S3501 has a much larger tank that holds up to 450 ml (15 oz.) versus 220 ml (7.4 oz.) on the S3101.

3, Power
The S3101 is 1,550 watts versus 1,200 watts on the S3501.

4, Handle
The S3501 has a telescopic handle that the S3101 does not feature.

5, Water Filling
The S3101 has the water filling hole at the back (near the top of mop base) whereas the S3501 has it at the front (middle).

P.S.
Download user guides
Shark S3501 Manual – http://www.sharkclean.com/include/pdf/manual-S3501.pdf
Shark S3101 Manual – http://www.sharkclean.com/include/pdf/manual-S3101.pdf

There are some notable differences between Shark Steam Pocket Mop S3501 and Shark Lift Away Professional Steam Pocket Mop S3901.

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Steam Control
The S3901 features Shark Intelligent Steam Control that offers 3 options – Dust, Mop, and Scrub.
They are designed for light cleaning, everyday cleaning and tough cleaning.
Whereas the S3501 has no steam settings. You have to pump the handle in a downward motion to steam, the amount of steam you get depends on the speed you move the mop.

Design
The S3901 can be turned into a handheld steamer for cleaning and sanitizing above floor surfaces.
Whereas the S3501 does not have a detachable handheld unit, it only works as a stick mop. So you cannot use it to clean counter top or similar spots.
The S3501 has a telescopic handle so you can adjust it to fit your height whereas the S3601 handle has a fixed length.

Water Capacity
The S3501 has a larger water tank that holds 450 ml (15 oz.) versus 300ml (10.14 oz.) on the S3901.

Power
The S3901 is 1,550 watts versus 1,200 watts on the S3501.

Included Accessories
The S3901 includes more accessories. It comes with a Rectangular Mop Head, 1 All-Purpose Scrub Pockets, Accessory Hose, Concentrator Nozzle, Flat Scrubber Tool, Garment Steamer with Brush, Garment Microfiber Bonnet, All-Purpose Flat Scrubber Pad, and Filling Flask.
Whereas the S3501 comes with a Rectangular Mop Head, 1 Shark Washable All-Purpose Pocket, and Filling Flask.

If you know any other difference between the Shark S3501 and S3901 steam mops, please share below.

P.S.
Download user guides
Shark S3501 Manual – http://www.sharkclean.com/include/pdf/manual-S3501.pdf
Shark S3901 Manual – http://www.sharkclean.com/include/pdf/manual-S3901.pdf

images from manufacturer

The key difference between the Shark S3601 and S3901 steamer is that the S3901 has a detachable design which allows it to transform from a stick into a handheld steamer for above floor cleaning, whereas the S3601 is just a stick steamer, it does not have a removable handheld unit.
So, if you want to steam clean and sanitize hard floors as well as above floor surfaces, such as counter tops, windows and upholstery, the Lift Away S3901 meets your needs.

The second notable difference is the water tank size. The S3601 has a larger water tank which holds 500ml (16.9 oz.) whereas the S3901 has a water tank capacity of 300ml (10.14 oz.).

Another difference is the included accessories. Because of the removable handheld design, the S3901 has accessories that the S3601 does not come with, such as the Accessory Hose and Flat Scrubber Tool.
The S3601 comes with a Rectangular Mop Head, 1 Shark Washable All-Purpose Scrub Pocket and a Filling Flask.
The S3901 comes with a Rectangular Mop Head, 1 All-Purpose Scrub Pockets, Accessory Hose, Concentrator Nozzle, Flat Scrubber Tool, Garment Steamer with Brush, Garment Microfiber Bonnet, All-Purpose Flat Scrubber Pad, and Filling Flask.

Both of them has 3 levels of steam settings and use Only Water.

If you know other difference between the Shark Professional Steam Pocket Mop S3601 and Shark Lift Away Professional Steam Pocket Mop S3901, please share below

P.S.
Download manuals to learn details about these 2 Shark Steamers.
Shark S3601 User Guide – http://www.sharkclean.com/include/pdf/manual-S3601.pdf
Shark S3901 User Guide – http://www.sharkclean.com/include/pdf/manual-S3901.pdf

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Cord Length
The Shark Professional Steam Pocket Mop S3601 and S3601D has a 22 feet cord whereas the S3601co has a extra long cord 30 feet cord.

Includes
The S3601 comes with 1 Rectangular Mop Head, 1 Washable Micro Fiber Pad and a Filling Flask.
The S3601D comes with 1 Rectangular Mop Head, 1 Triangular Mop Head, 2 Washable Micro Fiber Pads and a Filling Flask
The S3601CO comes with 2 Rectangular Mop Heads (one wider and one narrower), 1 Triangular Mop Head, 4 Washable Micro Fiber Pads and a Filling Flask

Color
The S3601 and S3601D are whereas the S3601CO is purple.