These 2 Bissell carpet spot and stair cleaners are similar except for some small things.
They have hand tool with flex hose to clean spots. They have dual tanks to keep clean and dirty water separate, but with NO built-in water heater, it is recommended to use hot tap water. There is also no separate detergent tank for holding detergent, thus you cannot control how much detergent to use during cleaning process, instead, you have to mix clean water with detergent before cleaning.
Below are some differences.

Bissell 5207F vs 1400B
images from manufacturer

Heatwave Technology
The SpotClean Proheat features the Heatwave Technology that maintains water temperature whereas the Little Green does not have this feature.

Tank Capacity
The Little Green has larger tank capacity which is 48 oz versus 37 oz on the SpotClean Proheat.

Included Accessories
The SpotClean Proheat comes with a 3-inch Tough Stain Brush and a DeepStain Tool whereas the Little Green only includes the 3-inch Tough Stain Brush.
The SpotClean Proheat comes with 2 bottle of trial size formula whereas the Little Green includes one bottle.

Warranty
The Bissell SpotClean Proheat 5207f carries 2 year limited warranty whereas the Little Green is backed by one year limited warranty.

According to customer rating, the Little Green might be a better choice. If you need a Bissell spot cleaner with larger tank capacity, you can try the Bissell SpotClean Professional 3624 which has a much larger tank and a longer cord.