I prefer the 50 minute lectures. Many theorems fit nicely into these smaller lectures. These lectures are easier to prepare. But most importantly I remember struggling to keep focused as a student in those longer lectures and I don't want to subject my students to the same.
There are variations on the theme. I took a graduate cryptography class with Silvio Micali that went for three hours once a week. We did have a muffin break in the middle and Silvio has the personality to pull it off.
During my sabbatical year in Amsterdam I taught a short course that had 90 minute lectures. The students insisted on having a break in the middle. Most Dutch movies theaters inserted an intermission in the middle of movies. Apparently the Dutch have an attention span no longer than half of a soccer game. My kind of people.