At my old uni, I used to teach a lot of Practical English (Conversational English/Freshman English) classes once/week to 7-9 different classes of students. Each class was 1.5 hours long.
And where I'm working now, it's kind of the same deal, except I have only 4 Practical English classes and I meet them twice a week for 1.5 hours each time.
Why I liked once/week? It's easy for the teacher. Obviously preparing only one lesson and teaching it 8 or 9 times is quite simple. But, on the downside, it can be tedious and boring by the time you get to round #9. However, you have a chance to learn from your mistakes in the early lessons about what didn't go so well and by the later lessons, you can actually be quite a fabulous teacher :) And, it's easy to keep things fresh when you only see students for such a short amount of time every week.
However, on the downside, it's not enough time for students to actually improve in their English skills, even with homework outside the classroom. And, it was often weeks into the semester before I actually learned student's names and got to know a bit about them. And also, it would often work out that a certain class would have 2 or 3 or 4 holidays during the semester. When you only have about 15 weeks with a class, missing 4 classes is a lot.
Teaching each class twice/week is more work for the teacher in terms of preparation. Basically, it's double. And, it's not as easy to plan activities that are new and interesting and exciting for the students. However, students can actually have a change to improve their skills. I'm finding it quite useful to teach a lesson in one class, and then have a kind of "review session" the next one to consolidate what the students learned from the last lesson. And, I feel like I have more freedom to have relaxed days, where I can try different activities, such as a Picture Quest, since I will see each class about 25-30 times. And, if there are a few holidays, it's not really a big deal, since it's quite rare that the class will miss both their days during the same week.
Anyway, to sum things up: I'm finding that I like teaching the same class twice/week a lot more than once!