1. Training (are the both relatively close in what they teach)
I'm not sure if anyone knows the answer to this. Most guess that the answer is "no." Of the three schools in question, two are close in what they teach. It is clearly stated on their websites and it is known to prepare students to get a job and keep it. Two of them use respected textbooks and have industry-respected instructors to facilitate and guide students.
2. Certificate vs. diploma
This is not important. They are the same thing. They just reflect the fact that you finished the course successfully. What does matter is that you do not think it is a certification as an MT. A certification can only be granted by AHDI after you successfully pass their exam.
3. 4 months for one program vs. 9-12 for another.
How many students actually do it in 4 months? Or 9? Will you have to pay extra if you need more time? Check the fine print--a extra payment every couple of months? Or does the school just seem to take longer because they're realistic about how long it will take? Maybe they recognize that people are different and are willing to work with you as long as it takes at no extra charge.
Some schools have a policy of cheap-appearing tuition, no instructors, and making students struggle on their own. It's the student's fault if they can't do it in break-neck speed. If the student passes the only exam in the course, the final, they pass.
Other schools have a policy of realistic tuition, full instructor support, and of leaving nobody to struggle on their own. The school feels responsible for student success. They work with students who need additional help and material. There are exams constantly, so the student is well-prepared to take employer tests and there is no surprise "sorry, you didn't pass" at the end.