6 of one half dozen of the other. I don't believe one is better over the other. back is an ambient style heat and belly is a direct style. Belly would likely be better in a cool room back maybe no so good in a cool room. In a warm room it likely doesn't make much difference both will work. In a failure of a t-stat back is likely safer as it simply will not get as hot. On the flip side it uses more power to produce the same temp. No clear choice here pick what makes the most sense to you.
Alex