The difference between Shark Light and Easy Steam Mop S3251 and S3601 Professional Steam Pocket Mop are as follows.
1, Steam Setting
The S3601 has 3 steam settings – Dust, Mop, and Scrub, they are designed from light cleaning delicate hard floors to scrubbing tough, stuck-on messes on hard surfaces.
Whereas the S3251 has no steam settings. To get steam out, you have to pump the handle.
2, Steamer Head and Pads
The head on S3601 can be flipped over, and it uses double-sided reusable micro-fiber pads. These can double the cleaning area of the steamer without changing pads. Whereas the S3251 head does not flip over for 2 sided cleaning.
The S3601 mop head is easily detached from the mop body by pressing a release button. This allows you to quickly switching between mop heads such as the rectangular mop head and triangular mop head.
While the S3251 seems to have to use a screw driver to detach the head.
3, Tank Capacity
The S3601 has 2X tank capacity.
The S3251 holds up to 250 ml (8 oz.) versus 500ml (16.9 oz.) on the S3601.
The S3251 has a telescopic handle that can be adjusted to fit user height whereas the S3601 handle is not telescopic.
Download user guides
Shark S3501 Manual – http://www.sharkclean.com/include/pdf/manual-S3601.pdf
Shark S3101 Manual – http://www.sharkclean.com/include/pdf/manual-S3251.pdf