Which one is easier on your back?
I don’t like bench press in general, since it doesn’t carrover much. but I think in your case it is a good alternative to the press since you have back issues.
I have problems with my shoulders from various injuries. Before crossfit, barbell bench caused pain, and dumbbell cased less pain.
Now they are both comfortable. They, nor any pressing movements are strong for me. Big time goats that I would like to make cabrito out of soon.
Both movements are effective. Barbell allows for higher weight, and arguable more strength gain. Dumbbell recruit more stabilzing muscles developing greater should stability.
This is nothing more that my observations of what happened for me.