It depends on what you want to meditate on. I often want to find solutions to my own feelings, therefore I meditate on my own. But if you just need to relax and calm yourself down, the guides on this app is perfect. As well as many other examples.
I prefer guided meditations, definitely. I'm not sure why, but meditating on my own usually leaves me feeling sick, dizzy, and very out of it – which isn't exactly the intended result of a meditation session.