Bissell 3624 vs 5207
images from manufacturer

All of these 3 Bissell portable spot cleaners have separate clean and dirty water tank, but they do not have water heater. You should fill the clean water tank with hot tap water mixed with detergent, then use the included hand tools with the flexible hose to clean spot and stains.

The Bissell SpotClean 5207a and SpotClean Proheat 5207f are the same machine, they only have slightly different tools.
The difference between SpotClean Pro 3624 and SpotClean 5207a / SpotClean Proheat 5207f are as follows.

Tank Capacity
The Pro version has larger tank capacity.

Heatwave Technology
The SpotClean and SpotClean Proheat feature the Bissell Heatwave Technology that directs the motor heat to maintain the water temperature during cleaning.
Whereas the Professional does not have this feature.

Motor Amps
The SpotClean and SpotClean Proheat have a 3 amp motor whereas the Professional uses 5.7 amps motor.

Cord Length
The Prefessional has longer power cord.

Tool
All of them come with 2 hand tools, however, the larger tool on the Professional model is a bit wider.

Weight
The SpotClean / SpotClean Proheat are lighter.

Pirce
The Professional is more expensive.

All of the carry 2 year warranty limited warranty and come with trial size cleaning formula.

Bissell Spotbot vs SpotClean
images from manufacturer

The Bissell SpotClean 5207a and Proheat 5207f are the same aside from color and included accessories.
The Spotbot 78R5 and Spotbot Pet 33N8a/33N8 are the same machine except for color and included detergent.

Both of the Bissell Spotbot and SpotClean are designed for removing spots and stains such as pet stains on carpets, rugs, stairs, upholstery, auto interiors, etc. But they are different from each other, here are some major differences between them.

Design
Bissell Spotbot/Spotbot Pet have a handsfree function that allows you to remove stains with a push of button. The machine has Spiral Brush on the bottom and also has preset cleaning cycles that spay solution, brush, and suck up water automatically. You just press a button, leave it on a stain, and it would clean the surface for you.
While the SpotClean/SpotClean Proheat does not have Spiral Brush, it only has hand tools that you can use to remove stains.

Tank
The tanks on both machines are small, however, the tanks on the SpotClean is larger.

Warranty
The Spotbot/Spotbot Pet carries 1 year limited warranty whereas the SpotClean/SpotClean Proheat is backed by 2 year limited warranty.

Size & Weight
The Bissell SpotClean/SpotClean Proheat is larger, but lighter.
Maybe this is because of the lack of spiral brush.

Heatwave Technology
The SpotClean/SpotClean Proheat features the Bissell Heatwave Technology which utilizes the heat that generated by the motor to maintain the temperature of the hot tap water you filled. Whereas the Spotbot/Spotbot Pet does not have this Heatwave Technology.
And both of them have no water heater, so you have to fill the clean tank with hot tap water.

Accessories
There are also difference on the included hand tools and detergents.
The Spotbot/Spotbot Pet comes with a 3″ Tough Stain Brush.
While the SpotClean/SpotClean Proheat also has the 3″ Tough Stain Brush and one larger hand tool for cleaning stains from stairs, upholstery and other hard-to-reach places.