It is truly amazing how kids react when you move them into a big bed. I moved my DD into a big bed at 18 months. Well officially we put a matress on the floor for two weeks then into the big bed. We didn't have a bed rail...just placed another mattress beside the bed in case she fell out, a soft landing, but helping her to understand the need to stay in the bed.

She always moved around the cot during sleeps but as soon as she moved to the single bed, she was fine, didnt move at all. And she only ever fell out ONCE.

DS was born when DD was 22 months - so it gave her 4 months to get used to it.

I'd suggest a bed over a cot