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Sr 2 IrO 4 and Sr 3 Ir 2 O 7 form two families of spin-orbit Mott insulators with quite different charge gaps and an antiferromagnetic (AF) ground state. This offers a unique opportunity to study the impact of long-range magnetic order in Mott insulators. It appears to play a different role in the two families, as there is almost no change of the resistivity at the magnetic transition T N in Sr 2 IrO 4 and a large one in Sr 3 Ir 2 O 7 . We use angle-resolved photoemission to study the evolution of the spectral lineshape through the magnetic transition. We use Ru and La substitutions to tune T N and discriminate changes due to temperature from those due to magnetic order. We evidence a shift and a transfer of spectral weight in the gap at T N in Sr 3 Ir 2 O 7 , which is absent in Sr 2 IrO 4 . We assign this behavior to a significantly larger coherent contribution to the spectral lineshape in Sr 3 Ir 2 O 7 , which evolves strongly at T N . On the contrary, the Sr 2 IrO 4 lineshape is dominated by the incoherent part, which is insensitive to T N . We compare these findings to theoretical expectations of the Slater vs Mott antiferromagnetism within dynamical mean field theory. [ABSTRACT FROM AUTHOR]