Sleep on stomach or back?

My brother is 11 years old.. when he was a baby, my mom got him this little thing that keeps him propped up on his side because they recently said that putting babies on their backs reduces the risk of SIDS. However, she was worried about him spitting up and choking on it while he was sleeping.. so she found the little “side-prop” tool. Which do you think is safer? Back or stomach? Or side? My baby is due next month and all I keep thinking about is her choking on her own vomit in her sleep.