safety dilemma.

My almost 4 month old daughter is refusing to sleep in her bassinet UNLESS she's on her tummy she wakes up and cries if she's on her back but when we put her on her tummy she sleeps soundly for hours or she will sleep soundly in our bed. Now I'm faced with the dilemma of is it safer for her to sleep alone and on her tummy or with us and in her back.... what do you guys think...?