I'd go with the divan or make sure you have a very good quality mattress if you go slatted - i once had a thin mattress on slats (cheap rented room at uni) and if I got up the wrong way I would lose a leg down the slats!!! Apparently my less curvy housemates didn't have this problem!!
I'm not in the position of being able to suggest the best type of bed for heavier people really, but I had a friend who weighed over 40 st.
A wooden slatted base was out of the question, but maybe the metal ones aren't.
My friend bought a good quality divan, which had an equally good memory foam mattress on top. This prevented her from rolling towards her partner.