my girl used to sleep on the mattress. Now she's sleeping on an adult-sized bed.
I find it very safe when she was sleeping on the mattress cos even if she could roll over, she wouldn't really fall hard.
Now that she's on the bed, i tend to worry a bit more and have to put barriers on the side. I think I'll soon have to get a bed barrier.
$ wise, I feel that it's more worthwhile to buy an adult-sized bed (hers is extendable to king size) so that she can use it till she's in her teens or even later.
and i can sleep with her when i bf her.