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