Retainers after treatment - Will a fixed one hold them in place, or would I be better off getting a removable one?

I've just started tray 20 of 21 so I've got 4 weeks left of treatment. My dentist asked me what sort of retainer I wanted and I told him that I wanted a fixed one, but from reading reviews, I'm now unsure. Will a fixed one hold them in place, or would I be better off getting a removable one? Does this just need to be worn at night?

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