The regular Momma is for blacksmith and farrier forging. The end doors do open, but that won't work well for most knife forging.
The "Momma with end ports" is just a two-burner version of the Lowboy. For the extra $100 the Lowboy is far more versatile.
The Knifemaker and Lowbow are for knives and similar forging.
IMHO, for a hobby knifemaker/blacksmith doing normal knives, tomahawks, plant hangers, and fun decorative, projects, the three-burner Lowboy is far better for knives and general forge projects that the two-burner Knifemaker. The Knifemaker is a bit longer but limited in width. It has an open port in one door, but you would have to open the other end completely to work a longer blade. It also does not allow placement of the blade at much of an angle to keep the tip out of the flame. The Lowboy has double ports and enough length for 90% of knives. The wide floor allows multiple blades or billets to be worked, and placement of the blade to avoid overheating the edge and tip. This is very important when doing HT in a forge. If working a longer blade you pass it through the back port. You can work any size of axe or hawk in the large floor area. If an occasional horseshoe is your thing, it will do that too. I believe the Lowboy was created to be a multi-task forge.
The number one NC forge for knives, if you really plan on doing larger blades and welding billets, is the Knifemaker 21. The Knifemaker 21 is a beast. If doing swords and using the back door in the open position, you can cut some soft firebrick to partially close the 4X4.5" port to 3X2". I had one of these and gave it to a relative. If knives/swords/damascus are all you will do, then the extra $100 is a no brainer.
A good plus for buying a NC forge is the resale value. You can usually get a high percentage of the purchase price if you go elsewhere in your hobbies. They are not the cheapest forges available, but ... again, IMHO .... probably the best for the money. Buy once ... Cry once ... Smile a long time.
I hope this helps - Stacy