The Bissell Little Green Machine series includes a few models:

• The original Little Green such as 1400B;
• Little Green Pet such as 1400W (exclusive to Walmart);
• Little Green Pet Deluxe such as 3353;
• Little Green ProHeat such as 2513G and 5207G (exclusive to Target);
• Little Green Pro such as 2505 (exclusive to Walmart);
• Little Green Pet Pro such as 2891.

They are portable carpet cleaner machines designed for removing spots and stains on carpeted stairs, area rugs, upholstery, auto interiors, etc. (If you want a deep cleaning carpet extractor for large wall to wall carpet, you may want to consider the Bissell BG10 carpet washer.)
Though they are all from the Little Green series, they are not exactly the same, they have some similarities as well as some differences.

Below I will list some differences between them to help you make an informed purchase.

• The original Little Green (1400B/1400M) , 3369, the Little Green Pet(1400W), Little Green Pet Deluxe 3353 are almost the same except for the color and included formulas and tools.
  The Little Green 1400B/1400M are in green color, 3369 is dark grey, the Pet is in purple, Pet Deluxe 3353 is dark grey/blue.
  The Little Green comes with one trial size general spot and stain formula while the Pet, Pet Deluxe comes with 2 trial size bottles of formulas that are produced specifically for removing pet stains and odors.
  The Little Green comes with only one tool: 3-inch Tough Stain Tool, whereas the Pet and Pet Deluxe includes an additional Stain Trapper Tool.
  Otherwise, these machines are almost the same.

  3-inch Tough Stian Tool: Cleans up spots, stains and other messes on carpeted stairs, upholstery, car seats, and other hard to reach surfaces.

  Stain Trapper Tool:
  

  

  image from Bissell infomercial

  
  Unlike the other tools, this self contained tool has its own container, you use it to spray, scrub and suction up messes, and those messes will be collected and stored inside the tool.

• The ProHeat is a bit different from the above 2 models. It has a different shape and features the HeatWave technology.
  There used to be an old ProHeat version such as model 14259, the shape is similar to the original Little Green. It has a 9 amps motor and a built-in heater to continuously heat the water in the tank up to 25 degree hotter and the temperature will not be over 180 degrees. It also has a Heater Switch and Heat Indicator Light at the front besides the Power Switch. According to the info on Bissell official website, this one is no longer available now.
  The new ProHeat version, such as 2513G and 5207G, now equipped with a 3 amps motor, and they no long have a heater inside the machine. Instead, they feature the so called HeatWave technology – it utilizes the heat that generated by the motor to maintain the water temperature, but it does not heat up the water like a heater.
  And the appearance is completely different from the older version. It looks more like the Bissell SpotClean series.
  This one comes with a 3-inch Tough Stain Tool, a 3-in-1 Stair Tool, a HydroRinse Self-Cleaning Hose Tool, and a trial-size Professional Spot & Stain + Oxy formula.

  3-in-1 Stair Tool:
  

  

  image from Bissell infomercial

  
  This tool sprays, scrubs and suctions up dirt and stains from the outer curves, corners and treads of carpeted staircases.

  HydroRinse Self-Cleaning Hose Tool:
  

  

  images from Bissell infomercial

  
  This tool is designed specifically to clean out your hose after each use.

• The Little Green Pro, such as 2505, and Pet Pro, such as 2891, are two upgraded versions. It seems that these two are the same machine as the SpotClean Professional 3624, but using different names. They are more powerful and larger than the Little Green, they have 5.7 amps motor, longer power cable which is 20-foot long versus 15-foot on the previous Little Green models, longer hose which is 5-foot, double tank capacity which is now 96 ounces versus 48 ounces on the Little Green, weighs over 3.5 pounds heavier, and carry one more year limited warranty than the original Little Green.
  The main difference between these two is the color, included tools and formulas.
  The Pro is in Black/ChaCha Lime while the Pet Pro is Black/Blue.
  The Pro comes with a Deep Stain Tool, a 4-inch Tough Stain Tool and a trial-size PRO MAX Clean + Protect Formula. Whereas the Pet Pro includes a Pet Stain Tool, a 3-inch Tough Stain Tool, a Spraying Crevice Tool, and one trial-size PET PRO OXY Urine Eliminator formula.

  Deep Stain Tool: It delivers formula deep into the carpet fibers, and its rubber needles and brush bristles help lift and remove those tough messes buried deep within carpet fibers from the bottom up.

  4-inch Tough Stain Tool:
  Similar to the 3-inch tough stain tool, but has a wider cleaning path, scrubs and cleans up tough stains on carpeted surfaces, upholstery, car seats, etc.

  Pet Stain Tool:
  

  

  image from Bissell infomercial

  
  Similar to the Deep Stain Tool, delivers formula deep into the carpet fibers to lift and remove pet stains and odor.
  Spraying Crevice Tool: helps tackle stains in tight spaces and other hard to reach spots.

Visit bissell.com to download user guides to read more details about these portable carpet cleaner machines. To save you time, I list the links to the PDF manuals download pages below:

Little Green
https://support.bissell.com/app/answers/list/p/67268/c/26115
Pet
https://support.bissell.com/app/answers/list/p/67268/c/26115
ProHeat
https://support.bissell.com/app/answers/list/p/171831/c/26115
Pro 2505
https://support.bissell.com/app/answers/list/p/67256/c/26115
Pet Pro 2891
https://support.bissell.com/app/answers/list/p/67256/c/26115

If you know other difference between them, please share below to help others make a informed decision.