On stable branch updates come in bulk every 2-4 weeks. On testing branch every couple of days (recently more like once a week), on unstable branch every day (direct packages from Arch, delayed few hours or more).
This way, if there are any major problems, they are caught in testing or in unstable. Stable users have better experience and no surprises, because on unstable branch, things could stop to work every day. There is a reason why stable branch is called stable .
There are huge differences in branches and on unstable it's really feels like a Russian roulette - you have to be constantly vigilant what is happening on Arch branch, because chances of breaking something major are high. Testing branch is much safer and stable, but again, some surprises come up once in a while. Stable branch gives you piece of a mind for the most time.
If you get to know Manjaro later, you can easily switch branches whenever you want or need, but for now, get to know Manjaro in its stable version.