I don't know how adequate the 10-minute sessions promised by apps like Headspace are in the long run. In my experience, a 20-minute session feels a lot more effective than 2 10-minute sessions, and so on for 40 minutes over 20 minutes, and so on. I found Headspace and similar apps to be a good introduction to meditation for the first week or two, but they really are 'meditation lite' compared to reading some of the c…
I think it depends on your individual needs if two 10 minute sessions or one 20 minute session is better. Some people gain concentration after 10 minutes, some people get impatient. Longer sessions might give you more concentration, but more frequent meditation might give you more mindfulness in your daily life.
In my mind the biggest disadvantage of headspace is that there is no community around it. It helps a lot to meet in person and practice together.